This article contains spoilers from One Piece's Elbaf Arc. Luffy's mastery over Gear 4 allows him to use it in specific body parts, improving stamina and Haki. Luffy's ability to freely switch ...

After learning Haki, Luffy managed to combine it with his Devil Fruit powers, and ended up creating Gear 4, which works on the concept of inflating muscles, as Luffy calls it muscle balloon.