Gear Fourth Boundman is an immensely powerful transformation technique used by Monkey D. Luffy in the anime series One Piece. In fact, it is part of his Gear Fourth, combining power from his Gomu Gomu no Mi Devil Fruit with advanced applications of Haki. Besides, Boundman focuses on strength and elasticity, which makes it a very powerful combat form.
The English name "Boundman" is literally translated from the Japanese term, meaning a man who is bound by a rope or something else so he cannot move. The naming convention reflects his appearance in that form—his body seems to be constrained and bulging with power.
Luffy first revealed Gear Fourth Boundman during the fight with Donquixote Doflamingo in the Dressrosa Arc. The dramatic reveal of it happened in Episode 726 of the anime. In terms of its release, this form represented an incredible leap in power for Luffy, showing his growth and ingenuity in devising new ways to harness the potential of his Devil Fruit.
Visually, Boundman is striking and distinct. Luffy's body changes dramatically when it's activated. His chest becomes bodybuilder-like while his arms and legs become very muscular but disproportionately small. This physique adds to the fighting style of Boundman, allowing powerful, springlike attacks.
Boundman has loads of abilities within this form, making him pretty impressive. Luffy would be able to compress his fists into his arms, which he could then release with explosive force to bring out devastating attacks. He could bounce himself off the air itself; that is what adds up the unpredictability to the moves, making him hard to track for any opponent.
Firstly, most of the fans thought that this was the only form Gear Fourth had. It was revealed, though, that in reality there were many versions of Gear Fourth with their strength and application. Boundman seems to be, though the most famous and frequently used form.
Among the important features attributed to Boundman is that he has incorporated Haki. In this form, Luffy covers his body with Armament Haki, which gives him offensive and defensive powers. Combining his rubber powers with Haki empowers Luffy to fight against the strongest foes in the One Piece world.
Even with its more than immense power, Boundman does not come without its limitations. It puts an incredible amount of strain on Luffy's body and Haki reserves, and he tends to use both at great speed. Several times after using Gear Fourth, Luffy has needed a recovery period during which he is unable to use Haki.
With the introduction of Gear Fifth into the series, where this goes is that fan speculation has been rife regarding whether or not Gear Fourth has become obsolete. Boundman does have special properties and Luffy seems to be able to hold it for a relatively easy amount of time, so there's still situational utility that could be derived from it in later fights.