Gomu Gomu no Mi/Gear 4 Techniques

Gear Fourth: Boundman is a technique first seen in Luffy's battle against Donquixote Doflamingo. It was developed by Luffy during his training on Rusukaina.

Overview
In order to activate the technique, Luffy coats his arm in Busoshoku Haki and then bites into his forearm. Similar to how he activates Gear Third, he blows an incredible amount of air into his body, but this time he inflates his muscular structure. He then distributes the air throughout his body, with emphasis on his upper half.

In this form, his body's proportions become warped and is much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. The coating across his torso resemble the wispy ends made from flames, giving it the appearance of tattoos, and he also gains shading around the inner and outer edges of his eyes. As a side-effect he is incapable of standing still on the ground, and instead constantly bounces on the spot. This form was created in order to combat the many powerful and large animals on Rusukaina. All-in-all, this form (hardened Busoshoku Haki not included) resembles his appearance when he became Nightmare Luffy.

When using Gear Fourth, Luffy's physical strength and speed is increased to the point where he is able to overwhelm a member of the Shichibukai, Donquixote Doflamingo, whereas beforehand both Gear Second and Gear Third were unable to do much damage. Luffy also shows a much more versatile side of the Gomu Gomu no Mi's powers: using his elasticity to achieve a form of pseudo-flight, changing the direction of his attacks mid-strike to hit a moving opponent, and strengthening his attacks in a similar manner to the Bane Bane no Mi by compressing his arms and legs like springs.

According to Doflamingo, the power of his attacks increases greatly in this form. His durability is also enhanced, as despite being coated in Busoshoku Haki his body is still rubber, and as a result Doflamingo's kicks simply bounce off of his body.

This form, much like Gear Second, seems to utilize the principles of Rokushiki. It possesses both high speed movement reminiscent of 'Soru' and the ability to kick off the air, similar to 'Geppo,' cleverly adapted to work even better with his body's elasticity, not just bouncing off the air but allowing his punched and attacks to bounce off his own body. His body is also now strong enough to defend against cutting attacks head-on.

Techniques Utilized

 * Kinniku Fusen (筋肉風船): By coating his arm in Busoshoku Haki and biting into his forearm, Luffy is able to blow a large quantity of air into his body and inflates his muscular structure with the air blown into his arm spreading out to the rest of his upper body. In this form his durability is enhanced to the point where a kick from Donquixote Doflamingo, a Shichibukai, bounced back harmlessly, due to his body retaining its rubber properties despite being coated in Haki.
 * Gomu Gomu no Kong Gun (ゴムゴムの猿王銃（コングガン）): By compressing his fist back into his enlarged forearm, Luffy is capable of unleashing a devastating short-range punch with enough power to break through Doflamingo's own Haki-enhanced defense and blow the Shichibukai all the way from Dressrosa's palace to the middle of the city.
 * Gomu Gomu no Rhino Schneider (ゴムゴムの犀榴弾砲（リノシュナイダー）): Luffy, after compressing both legs into his body, delivers a double flying kick. This was first used as a surprise attack against Doflamingo. It has enough power to send the opponent crashing through dozens of buildings and across a town center.
 * Gomu Gomu no Culverin (ゴムゴムの大蛇砲（カルヴァリン）): Delivered like a regular Gomu Gomu no Pistol, even if dodged, Luffy can redirect his punch an unspecified number of times to still land with tremendous force. A culverin is a relatively simple firearm, ancestor of the musket and cannon.

Trivia

 * The appearance of this technique is strikingly similar to a previous form Luffy took, Nightmare Luffy, due to his increased muscle size and sinister expression.