He coats his body in a dense layer of Armament Haki to maintain the tension and shape of the air within his muscles.

Here’s a review of the episode where Luffy first transforms into — Episode 726 (One Piece, Dressrosa arc).

The Luffy Gear 4 transformation is a significant and iconic moment in the popular anime and manga series, One Piece. This transformation occurs during the Dressrosa Arc, specifically in episode 746 of the anime, titled "A New Chapter! Luffy's Final Gear 4th! Red Hawk on the Field of Dreams!"

Monkey D. Luffy ’s is a watershed moment in One Piece , marking the first time the future Pirate King utilized a form that combined advanced Armament Haki with the high-pressure elasticity of his Devil Fruit.

Not in the same episode (that’s 727–728), but 726 ends with Luffy compressing his arm and declaring “Gomu Gomu no…”, leaving you on the edge of your seat.