Luffy bites into his left forearm and begins by blowing air into his body’s muscle structure—not just his bones (like Gear Third). The result is terrifying and bizarre:

The name "Gear Fourth: Boundman" comes from its signature trait: . Luffy can now compress his rubber limbs into his body and release them like a pneumatic piston, resulting in:

Luffy first uses in Episode 726 of the One Piece anime, titled "Gear 4! The Phenomenal Boundman!". This debut occurs during the climax of the Dressrosa Arc during his high-stakes battle against Donquixote Doflamingo. Key Gear 4 Episode Guide

When Doflamingo tried to kick Luffy, he was bounced back. When he tried to slice him, the blade glanced off the bouncy tension. Luffy explained it best:

Episode 726 – Luffy inflates his muscular structure and coats his body in Armament Haki to gain a balance of flight, speed, and overwhelming power.