Rokushiki/Tekkai

We've trained our bodies such that we can turn them into "Iron Carapaces" if we so choose...

Tekkai is one of the six techniques called "Rokushiki".

Overview
The Tekkai hardens the users' muscles to the level of iron, in order to nullify damage taken from attacks. However, it can be broken by strong enough forces such as Luffy's various Gears, Zoro's Shishi Sonson and Tatsumaki, Sanji's Diable Jambe and Franky's Strong Hammer.

When Tekkai is active, the user is unable to move, with the exception of Jabra's Tekkai Kenpo. Tekkai can also be used to enhance the users' attacks by the increased density one gets from using Tekkai, similar to Busoshoku Haki.

The strength of Tekkai can vary for users with different physical conditions, as a person with a higher level of muscular strength would be able to take stronger forces and enhance attacks with his/her use of Tekkai. Furthermore, despite the increased physical density that one gains when activating this move, one should remember that the user's body is still flesh and blood. Thus, its resistance to other than solid attacks remain pretty much the same, making the user as vulnerable as ever to certain attacks. An example was during Sanji's fight with Jabra where he used the burning force of friction to incinerate through the latter's Tekkai or when Franky used his actual iron fist to break through Fukuro's Tekkai. As such, even masters of Tekkai are far from indestructible, though Rob Lucci was able to resist even cannonballs at the age of 13.

Variations

 * Tekkai "Utsugi" (鉄塊「空木」): Used by Lucci. This form of Tekkai is used entirely as a counter instead, where the purpose is not to absorb damage from the attack, but to transfer the shock of the attack back to the attacker, which can lead to broken bones in a normal human body. In the FUNimation dub and Viz Manga, this is called Iron Body Empty Tree.


 * Tekkai "Mushikaku" (鉄塊「無死角」): Used by Kaku in his half-giraffe form. Kaku bends his neck and legs to shift himself into a cube. When Tekkai is used in this form, he eliminates the weak points of his body, particularly his neck, and ensures that not a single spot is vulnerable to his following Kiri Shigure, though he looks ridiculous to his opponents. The pun is that "mashikaku", means "perfect square". In the FUNimation dub and Viz Manga, this is called Iron Body Edgewise.


 * Tekkai Kenpo (鉄塊拳法): Used by Jabra. Using Tekkai Kenpo, Jabra is able to move while using Tekkai. Tekkai Kenpo is also an entire fighting style, with various techniques of its own. In the Viz Manga, this is called Iron Body Kung Fu.
 * Tekkai Kenpo "Okami Hajiki" (鉄塊拳法「狼弾」): After hardening his hands with Tekkai, Jabra punches his opponent. In the Viz Manga, this is called Iron Body Kung Fu Wolf Bullet.
 * Tekkai Kenpo "Roga no Kamae" (鉄塊拳法「狼牙の構え」): Simply called Rōga for short. Rōga is one of Jabra's forms of Tekkai Kenpō. The way that Rōga is called is a pun in Japanese; oftentimes sentences end with darou, approximately meaning "probably". Sometimes, however, the particle ga would be added onto the end of a sentence to make it seem more direct. This is the way Jabra normally speaks, so he simply replaces where he would normally say darou ga (だろうが) with da rouga (だ狼牙). In the Viz Manga, this is called Iron Body Kung Fu Wolf Fang Stance.
 * Tekkai Kenpo "Roba no Kamae" (鉄塊拳法「狼芭の構え」): A form of Tekkai Kenpō, in which Jabra seems to combine Tekkai with Soru in order to move at extreme speeds. In the Viz Manga, this is called Iron Body Kung Fu Wolf Fang Stance.
 * Rokaru Area Network (狼狩エリア・ネットワーク): While moving at high speed, Jabra slashes at his opponent with both hands. Because of his high speed, it appears as if he was attacking with multiple hands. "Roukaru" is the Japanese way of pronouncing the word "local", making this attack a pun on "local area network". In the Viz Manga, this is called Wolf Hunt Highspeed Scratch.
 * Tekkai Kenpo "Don Poro" (鉄塊拳法「重歩狼」): A punch to the opponent's stomach with Tekkai in use. "Dongpo Rou" can also mean simmered square pork meat in Chinese. In the Viz Manga, this is called Iron Body Kung Fu Don Po Ro.
 * Tekkai Kenpo "Matenro" (鉄塊拳法「摩天狼」): Jabra flips onto his hands, then kicks his opponent up into the air. In the anime, this attack was portrayed as a jumping kick instead. "Matenro" is also Japanese for "skyscraper". In the Viz Manga, this is called Iron Body Kung Fu Devil Wolf.


 * Tekkai "Rin" (鉄塊「輪」): Used by Blueno. Blueno uses Tekkai and kicks one leg into the ground and starts rotating his legs at a fast speed, causing his legs to simultaneously slam into the ground on a set path toward his opponent. It also seems to incorporate Rankyaku, as the legs leave blue traces in the air like a normal Rankyaku shockwave and a glancing attack cut a small amount of Luffy's hair. In the Viz Manga, this is called Iron Body Wheel.


 * Tekkai "Sai" (鉄塊「砕」): Used by Blueno and Kibin. After using Geppou to project himself towards his opponent, Blueno uses Tekkai to harden his body, smashing the enemy. In the Viz Manga, this is called Iron Body Smash.


 * Tekkai "Go" (鉄塊「剛」): Used by Blueno and Kumadori. Visually, the move differs from the ordinary Tekkai in that the user visually tenses his body even further while standing still; the ordinary Tekkai could be activated seamlessly. According to Blueno, it is his strongest form of Tekkai. In the Viz Manga, this is called Iron Body Strength when used by Blueno, and Solid Iron Body when used by Kumadori.


 * Tekkai "Sansetsu Lariat" (鉄塊「三節ラリアット」): Used by Shuzo. Shuzo throws his arm back, hardens his body with Tekkai, and finally swings his arm forward. This attack is strong enough to counter one of Sanji's kicks, but its full effects are not shown due to it being deflected. The name of the technique most likely came from the fact that Shuzo's attacking arm has three sections due to it having two elbows instead of one.