This technique involves Luffy speeding up the blood flow in all or selected body parts, in order to provide them with more oxygen and nutrients. This makes him much faster and more powerful. The technique uses even more oxygen and energy, using up more food and causing Luffy to pant heavily. The increase in his metabolism causes him to be hungrier than usual or even have a paralysis type effect after prolonged use of it, like in the second fight with Rob Lucci, where Luffy was only able to lie on the floor. Lucci, one of the first of Luffy's opponents to witness it, compares the process to doping. Doping is adding more red blood cells to the body part/s, which in turn allows for greater delivery of oxygen and nutrients to the body part/s. In Luffy's case, his organs and blood vessels are made of rubber allowing them to expand and contract at greater rates without tearing. Essentially Luffy increases the blood flow by forcing more blood through his blood vessels thus increase his fighting ability. This mode's enhancements to Luffy's physiology giving him the ability to use a new, stronger set of techniques which are generally the same as his old ones, but are so fast that even a skilled assassin would have a hard time actually seeing them. He developed this ability after encountering CP9, and watching them use Soru. This is called Gear Two in the Viz Manga, Second Gear in the official simulcast subs and in the FUNimation dub. After seeing Luffy use Gear Second and understanding how it works, Rob Lucci speculated that using this technique would likely reduce his lifespan.
I've thought of a way to fight with all my strength so I won't lose to anybody! So I won't ever have to lose anybody. You won't be able to catch up with me anymore. All of my techniques have evolved by one step.
— Luffy's determined answer to Blueno in their fight
Prior to the timeskip, Luffy would initiate this technique by using his leg as a pump to increase the blood flow in all of his body parts. As a result, all of his body parts become stronger and faster. The technique used more oxygen and energy, using more food and caused him to pant heavily during use. He has been able to heavily damage some of the strongest individuals in the series while this technique was active, such as Magellan and Blackbeard. When this technique was in effect, his entire metabolic rate went so high that his sweat vaporized rapidly making his body look as if it were steaming and, in the anime, his entire skin turns slightly pink and shiny from the increased blood flow. He also says almost all of his attacks in English as opposed to Japanese.
At first, Luffy could only use Gear Second for a short period of time, because the accelerated speed at which nutrients are used up depletes his body rapidly. The mechanics behind this enhancement thus also mean reduced strength and agility after prolonged use, as pointed out by Lucci when Luffy's attacks start losing their speedy edge because with all his nutrients running low, having a faster blood flow would be useless as there are no nutrients to use. Due to this, as also pointed out by Rob Lucci, Gear Second hacks away at Luffy's life force dramatically, increasing a chance of having a much shorter lifespan because he is starving his body, though Luffy rebukes it, saying that it does not matter as long as he saves his friends.
After Gear Second wears off, Luffy can recover quickly by replenishing his body's nutrients by eating meat. Later in the Impel Down Arc, Luffy demonstrated a more excessive use of his Gear Second ability without such exhausting costs like his fight with Rob Lucci, as well as being able to activate it at conveniently brief periods of time to save his stamina and strength reserves for longer fights. This is possibly because of Luffy's mastery over the form along with his desire to save Ace, and possibly the large amount of food he ate in Newkama Land at Impel Down.
While not in the anime and manga, in the 7th movie of the series which takes place before the Water 7 Arc, Luffy briefly and unknowingly activated Gear Second during his fight against Ratchet, the antagonist of the film, when the sheer pressure of the machine Luffy is forced to hold up unknowingly causes his legs to end up pumping his blood flow. He then defeated Ratchet with a Gomu Gomu no Jet Bazooka, although he thought that it was a regular Gomu Gomu no Bazooka and was even surprised at the amount of force it had produced.
After the timeskip, it appears that Luffy has refined his control over this technique as he has now been shown capable of activating it by swiftly pumping blood from any of his limbs without having to assume his original stance. Luffy's refinement of Gear Second lead to him using the ability in short, controlled bursts, activating it briefly in order to use individual ability-specific attacks. This push for brevity was likely done as a means of limiting the amount of strain that it places on his body, a point that was noted and exemplified on multiple occasions during the first half of the story.
With this new mastery over Gear Second, combined with Busoshoku Haki, he can now easily destroy a Pacifista and take down a fish-man as large as Wadatsumi with a single Gomu Gomu no Jet Pistol. Furthermore, using a combination of Haki and Gear Second, Luffy is able to punch opponents at with such speed and force that it causes his fist to ignite, bearing a similar effect to his brother's signature attack.
Most of the techniques in this mode are named the same as the regular attacks with "Jet" added after the "Gomu Gomu no" prefix, though often times, Luffy will omit this prefix when using several attacks in succession. The known named techniques that are used by Luffy so far in this form are as follows:
Soru (剃,Soru?, literally meaning "Shave"): With the enhanced metabolism, Luffy is able to perform the Rokushiki technique Soru, allowing him to move at disappearing speeds. His prowess of it has surpassed that of Blueno's. In the Viz Manga and FUNimation dub, this is called Shave. This was first seen being used against Blueno.
Gomu Gomu no Jet Pistol (ゴムゴムのJET銃,Gomu Gomu no Jetto Pisutoru?, literally meaning "Rubber Rubber Jet Pistol/Gun"): Luffy delivers a Gomu Gomu no Pistol so fast that the fist's movement is invisible, and it truly seems like he shoots his opponent, except that the resulting impact is not akin to an actual pistol, but to a small hand-held cannon and powerful enough to propel the unlucky recipient several meters away. The incredible speed both makes the strike far more powerful, and harder to dodge. When using this technique, he points his other hand out to aim at his target. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Pistol. This was first seen being used against Blueno.
Gomu Gomu no Jet Stamp (ゴムゴムのJETスタンプ,Gomu Gomu no Jetto Sutanpu?, literally meaning "Rubber Rubber Jet Stamp"): Luffy delivers a Gomu Gomu no Stamp, with the same effect as the Jet Pistol. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Stamp. This was first seen being used against Blueno.
Gomu Gomu no Jet Bazooka (ゴムゴムのJETバズーカ,Gomu Gomu no Jetto Bazūka?, literally meaning "Rubber Rubber Jet Bazooka"): Luffy charges at his opponent at high speed, and delivers an incredibly fast and strong Gomu Gomu no Bazooka. In the Viz Manga, FUNimation dub, and FUNimation subs, this is called Gum-Gum Jet Bazooka. This was first seen being used against Blueno. It was apparently powerful enough to break through Blueno's strongest Tekkai technique, Tekkai: Go, as well as Boa Marigold's Busoshoku Haki-enforced guard. During the battle against Moria in his Shadow's Asgard form, Luffy demonstrated being able to use this move to propel himself from mid-air, allowing him to perform this attack continuously.
Gomu Gomu no Jet Whip (ゴムゴムのJET鞭,Gomu Gomu no Jetto Wippu?, literally meaning "Rubber Rubber Jet Whip"): Luffy delivers a 'Gomu Gomu no Muchi, so fast, that only the slipstream is visible. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Whip. This was first seen being used against Rob Lucci in order to protect Franky from him. Unlike the original Muchi, the kanji is alternately read in English instead of Japanese.
Gomu Gomu no Jet Bullet (ゴムゴムのJET銃弾,Gomu Gomu no Jetto Buretto?, literally meaning "Rubber Rubber Jet (Gun) Bullet"): An extremely fast, close-range punch with very great force. When attacking head-on, the stretching of the original Bullet along with the resulting snapback seem to occur almost simultaneously at once, making it virtually inescapable. This was first seen being used against Rob Lucci. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Bullet.
Gomu Gomu no Twin Jet Pistol (ゴムゴムのツインJET銃,Gomu Gomu no Tsuin Jetto Pisutoru?, literally meaning "Rubber Rubber Twin Jet Pistol/Gun"): Luffy punches with both arms simultaneously. Visually, the move only differs from his normal Gomu Gomu no Bazooka in that instead of doing an open palm strike, he has clenched fists which leave two knuckle shaped imprints on his foe. As since the move hit so fast, the air from it really looks like Luffy is literally firing two pistols. This was first seen being used against Rob Lucci and is one of the few moves unique to Gear Second (unless the anime-only regular "Gomu Gomu no Twin Pistol" is included). In the Viz Manga and FUNimation dub, this is called Gum-Gum Twin Jet Pistol.
Gomu Gomu no Jet Gatling (ゴムゴムのJET銃乱打,Gomu Gomu no Jetto Gatoringu?, literally meaning "Rubber Rubber Jet Gun Battering"): This is Luffy's strongest technique in Gear Second. Luffy stands still, bends forward and moves his arms upwards at speeds which they can no longer be seen, leaving only jet streams in their place, very much like how the Jet Pistol looks. This results in an effect comparable to Foxy's Kyuubi Rush, except that Luffy does not need to make use of Noro Noro photons to slow down his target in order to accumulate the hits. The hits land so fast that, after a volley, they seemingly connect simultaneously. The circles of air bursts from the punches surrounding Luffy, resemble the many barrels of an actual Gatling gun. However, due to the large amount of energy it uses, Luffy cannot sustain it for long, although the time it is used is longer than the regular Gatling. During his fight against the combined efforts of Marigold and Sandersonia, Luffy demonstrated an omnidirectional version of this move by falling on his back, then, while lying down, launches a rapid barrage of powerful high-speed punches from the ground, effectively nullifying both sister's attacks. This was first used to defeat Rob Lucci. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Gatling.
Gomu Gomu no Jet Rocket (ゴムゴムのJETロケット,Gomu Gomu no Jetto Roketto?, literally meaning "Rubber Rubber Jet Rocket"): A Gear Second version of Luffy's Gomu Gomu no Rocket technique. After grabbing onto something with stretched out arms, Luffy instantly propels himself forward with blinding speed. This was first seen being used against Moria. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Rocket.
Gomu Gomu no Gigant Jet Shell (ゴムゴムの巨人のＪＥＴ砲弾,Gomu Gomu no Gigant Jetto Sheru?, literally meaning "Rubber Rubber Giant Jet Shell"): This was Luffy's strongest technique at the time. While he is still in Gear Second, Luffy goes into Gear Third with Hone Fusen. With the combination of Gear Second's high speed and Gear Third's air within his bones, Luffy launches his air filled body and rams into an opponent like a large cannonball. This move is quite reckless, as Luffy suffers both from the excess burn on his body from Gear Second, and the difficulty of moving his body from Gear Third. This was first seen being used against Gecko Moria while he is in his Shadow's Asgard form. In the Viz Manga and FUNimation dub, this is called Gum-Gum Giant Jet Shell. The literal kanji reading is "Kyodai no JET Houdan", with "houdan" being a Japanese term for a gun shell.
Gomu Gomu no Jet Migawari (ゴムゴムのJET身代り,Gomu Gomu no Jetto Migawari?, literally meaning "Rubber Rubber Jet Scapegoat"): Somewhat of a gag move where Luffy stretches his arms to grab a person (an ally, an opponent, or otherwise a simple bystander) from afar then pulls them close to where he is to block an attack that Luffy would otherwise be incapable of surviving on his own. Using his Gear Second's speed, Luffy can capture himself a shield very quickly. He has a normal (non-Gear Second) variant of this move where Luffy sends the recipient spinning towards the assaulter to preoccupy them as he makes his escape. This was first seen being used by Luffy on Buggy, using the latter as a human shield against Dracule Mihawk. This is called Gum-Gum Human Shield Jet in the Viz Manga and Gum-Gum Jet Human Shield in the FUNimation versions.
Gomu Gomu no Jet Lance (ゴムゴムのJET槍,Gomu Gomu no Jetto Ransu?, literally meaning "Rubber Rubber Jet Lance/Spear"): A Gear Second technique that first appeared in the anime, however time after, Luffy makes the same move with its official name. Luffy delivers a very fast Gomu Gomu no Yari that seems to connect instantly with his opponent. This was first seen in the anime being used against Blueno. Unlike the original Yari, the English name is read as "lance" instead of "spear", despite the two techniques having the same lone kanji (槍).
Gomu Gomu no Jet Missile (ゴムゴムのJETミサイル,Gomu Gomu no Jetto Misairu?, literally meaning "Rubber Rubber Jet Missile"): A Gear Second technique that first appeared in the anime. Luffy stretches and grabs onto an enemy's body. Then, he very quickly retracts, crashing into the foe (making it a stronger version of Gomu Gomu no Rocket). This was first seen in the anime being used against Blueno.
Gomu Gomu no Jet Axe (ゴムゴムのJET斧,Gomu Gomu no Jetto Akkusu?, literally meaning "Rubber Rubber Jet Axe"): A Gear Second technique that first appeared in the anime. Luffy stretches his leg vertically and then crashes it down on his opponent, unlike the usual version however, rather than merely smashing his foot into the enemy the speed at which it is done is so quick that rather than attacking the area directly in front of him, his Jet Axe strikes several meters in length, thus increasing its range. This was first seen in the anime being used against Lucci. Unlike the original Gomu Gomu no Ono, not only is the attack read in English, but it removes the "sen" (battle/war) kanji (戦).
Gomu Gomu no Jet Rifle (ゴムゴムのJET回転弾,Gomu Gomu no Jetto Raifuru?, literally meaning "Rubber Rubber Jet Rotating Bullet"): A Gear Second version of Luffy's Gomu Gomu no Rifle technique that first appeared in the anime. The winding of the original Rifle along with its snapback happening almost at the same time as Luffy speeds towards the target makes this move impossible to evade. This was first seen in the anime being used against Blueno.
Gomu Gomu no Jet Hammer (ゴムゴムのJETハンマー,Gomu Gomu no Jetto Hanmā?, literally meaning "Rubber Rubber Jet Hammer"): The Gear Second version of Gomu Gomu no Ozuchi, which was not used since Luffy's battle with Don Krieg. Luffy wraps his legs around his opponent and, at an extreme speed, smashes him into the ground. Unlike the normal version, Luffy does not twist, and then un-twist his legs during the execution of this technique. Although in the anime he twists his legs but does not untwist them. It was first used against Vander Decken IX while Luffy was tied up. In the Viz Manga and FUNimation subs, this is called Gum Gum Jet Hammer. Unlike its original version, it is written in katakana and read in English.
Gomu Gomu no Jet Mace (ゴムゴムのJET鉾頭棒,Gomu Gomu no Jetto Meisu?, literally meaning "Rubber Rubber Jet Halberd Head Pole"): Luffy kicks his opponent with extreme speed while falling and spinning. This was first used against Caesar Clown. In the Viz Manga, this is called Gum-Gum Jet Mace. The kanji's literal reading is "Bouzubou", which may be a personal kanji term for a mace/club.
Gomu Gomu no Jet Spear (ゴムゴムのJET槍,Gomu Gomu no Jetto Supia?, literally meaning "Rubber Rubber Jet Spear"): Simply a more powerful, Gear Second version of Gomu Gomu no Yari, only it is now read in English as "spear" instead of "lance". It was first seen being used on the floor in Luffy's fight against Monet.
After the timeskip, Luffy has improved his Gear 2nd techniques via imbuing them with Busoshoku Haki. All of these attacks follow the naming concept of a bird of prey (most commonly Hawk).
Gomu Gomu no Red Hawk (ゴムゴムの火拳銃,Gomu Gomu no Reddo Hōku?, literally meaning "Rubber Rubber Fire Pistol"): A Haki enhanced version of Gomu Gomu no Jet Bullet. After entering Gear Second, Luffy uses Busoshoku Haki to harden his arm entering in Gear Second Busō. He then stretches it back like Gomu Gomu no Bullet. When cocked back, his arm ignites, releasing a stream of fire. When the attack hits, it creates a fiery explosion even underwater and deals immense damage. The Redhawk is a type of revolver made by the gun manufacturer Ruger, and the kanji name of this attack (literal reading is "hikenjuu") pays tribute to Ace's Hiken (火拳銃 means both "Fire Pistol" and "Fire Fist Gun"); the anime furthers this tribute by showing a flash of Ace when Luffy used this technique. This was first used against Hody Jones in his transformed state after the air bubble engulfed Noah. However, chronologically, it was first shown in use, almost instinctively (Luffy was shown surprised after he made such a move), during the TV special 3D2Y, in Luffy's final fight against Byrnndi World. In FUNimation dub and subs, this is called Gum-Gum Red Hawk, while in the Viz Manga it is called Gum-Gum Fire-Fist Pistol Red Hawk.
Gomu Gomu no Hawk Gatling (ゴムゴムの鷹銃乱打,Gomu Gomu no Hōku Gatoringu?, literally meaning "Rubber Rubber Hawk Gun Battering): A Haki enhanced version of Gomu Gomu no Jet Gatling. Luffy uses Gomu Gomu no Jet Gatling while using Busoshoku: Koka on his arms to enhance the power of his punches. He first uses this against Caesar Clown, but it gets blocked by Monet using her Devil Fruitability to create a wall. It was originally simply known as Buso Koka: Gomu Gomu no Jet Gatling (武装硬化 ゴムゴムのJET銃乱打,Busō Kōka: Gomu Gomu no Jetto Gatoringu?, literally meaning "Armament Hardening: Rubber Rubber Jet Gun Battering"), but was renamed to go with the "Hawk" naming theme when Luffy used it against Donquixote Doflamingo. In the anime, he uses this against Bilić, but misses due to Elizabeth's speed. In the FUNimation subs, this was originally called Armament Hardening: Gum-Gum Jet Gatling and in the Viz Manga and Funimation subs it's now known as Gum-Gum Hawk Gatling. The kanji's literal reading is "Taka Juuranda".
Gomu Gomu no Hawk Rifle (ゴムゴムの鷹ライフル,Gomu Gomu no Hōku Raifuru?): A Haki enhanced version of Gomu Gomu no Jet Rifle. After entering Gear Second, Luffy uses Busoshoku Haki to harden his arm. He twists his arm to create a spinning punch and increase the power. This was first used against Chinjao at Corrida Colosseum, and was sufficiently powerful to make Chinjao bleed, despite the fact that Chinjao was using Busoshoku Haki on his stomach to minimize the damage. Unlike the prior versions, the "rifle" part is not written in kanji.
Gomu Gomu no Eagle Bazooka (ゴムゴムの鷲バズーカ,Gomu Gomu no Īguru Bazūka?): A Haki-enhanced version of Gomu Gomu no Jet Bazooka. Luffy hardens his arms with Busoshoku Haki and stretches them both backwards. The arms then deliver a great impact to the opponent after Luffy blasts them forwards. This was first used against Donquixote Doflamingo. In the Viz Manga this is known as Gum-Gum Eagle Bazooka. The 'Eagle' in the technique's name is possibly derived from the semi-automatic pistol, the IMI Desert Eagle. The literal kanji reading for "eagle" is normally "washi", or "shuu".
Gomu Gomu no Hawk Whip (ゴムゴムの鷹鞭,Gomu Gomu no Hōku Wippu?): A Haki-enhanced version of Gomu Gomu no Jet Whip. Luffy imbues his entire leg with Busoshoku Haki, and then swipes it towards the target as an elongated kick. This was first used against Donquixote Doflamingo. Though in the anime, Luffy used this move without entering Gear Second. The literal kanji reading is "takamuchi" or "you'ben", and just like Jet Whip the kanji is spoken/read in English unlike the original Gomu Gomu no Muchi.
Gomu Gomu no Hawk Stamp (ゴムゴムの鷹スタンプ,Gomu Gomu no Hōku Sutanpu?, literally meaning "Rubber Rubber Hawk Stamp"): A Haki-enhanced version of Gomu Gomu no Jet Stamp. Luffy imbues his entire leg with Busoshoku Haki, and then stretches his leg out and hits his opponent with the bottom of his foot, leaving an imprint or "stamp" of his sandal. This was first used against Charlotte Katakuri.
Gomu Gomu no Jet Bazooka (ゴムゴムのJETバズーカ,Gomu Gomu no Jetto Bazūka?, literally meaning "Rubber Rubber Jet Bazooka"): Luffy used the Gomu Gomu no Jet Bazooka with clenched fists instead of palms and imbues his arms with Busoshoku Haki. He first used this against Shuzo in the Z's Ambition Arc.
Gomu Gomu no Jet Cannon (,Gomu Gomu no Jetto Kyanon?): Luffy used the Gomu Gomu no Jet Bazooka with clenched fists instead of palms. He first used this against Don Chinjao in Corrida Colosseum to counter his Buto Kaiten.
Gomu Gomu no Jet Stamp Gatling (ゴムゴムのJETスタンプガトリング,Gomu Gomu no Jetto Sutanpu Gatoringu?, literally meaning "Rubber Rubber Jet Stamp Gatling"): Luffy delivers multiple Gomu Gomu no Stamp attacks with the same power and speed as if performing a Jet Gatling. This was first seen being used against Don Chinjao. Unlike the original Gatling and its variations, this is written in katakana.
Gomu Gomu no Hawk Bullet: Similar to the Jet Bullet, it is reinforced by Busoshoku Haki, for extra power. Luffy used this attack against Issho, when the Admiral blocked his escape.
Gomu Gomu no Hawk Axe: Similar to the Jet Axe, it is reinforced by Busoshoku Haki, for extra power. Luffy used this attack against Issho, when the Admiral blocked his escape.
Gomu Gomu no Hawk Twin Pistol: Similar to the Jet Twin Pistol, it is reinforced by Busoshoku Haki for extra power. This attack was first seen being used in battle with Charlotte Katakuri.
Gomu Gomu no Red Hawk Gatling (,Gomu Gomu no Reddo Hōku Gatoringu?, literally meaning "Rubber Rubber Fire Gatling/Gun Battering"). Luffy is first seen using this attack in screenshots for 'One Piece: Pirate Warriors 3. Luffy covers both his arms with Busoshoku Haki, and combines both "Gomu Gomu no Red Hawk" and "Gatling". However, the attack never made its way into the game.
Gomu Gomu no Eagle Storm (ゴムゴムの鷲暴風雨,Gomu Gomu no Īguru Sutōmu?, literally meaning "Rubber Rubber Hawk Storm (Rain)"). A variation of Gomu Gomu no Storm used only in One Piece: World Seeker. No apparent modification can be seen.
Gear Second is similar to the Dragon Ball series' technique, Kaioken, as both forms takes the body of the user to the limit increasing their power and speed. In fact, during one of the first times Goku uses the Kaioken, he emits steam from his body, like Luffy does with Gear Second. In the anime, the resemblance is even more noticeable as Luffy's skin turns pink because of the accelerated metabolism and Goku's skin turns pink because of the Kaioken red aura.
The Toriko series' technique Shivering, is quite similar to Gear Second in appearance as well.