: Luffy coats his forearm in Armament Haki , bites into it, and blows air directly into his muscles.

Luffy revealed that he coats his body in Armament Haki, inflates his muscles like a balloon, and then uses the Haki to keep the air trapped inside. This creates a rubbery armor that defies physics.

Monkey D. Luffy first reveals in Episode 726 , titled " Gear Fourth! The Phenomenal Boundman!

Doflamingo, initially laughing at the comical sight, quickly realizes he made a mistake. Luffy’s voice drops, becoming deep and serious.

"The Climax of the Fierce Battle! Gear Fourth vs. Doflamingo" Episode Number: 726 Arc: Dressrosa

The genius of the episode was explaining the mechanic. It wasn't just inflation; it was .