Luffy coats his arm in Haki before biting into his forearm. Similar to how he activates Gear Third, he blows an incredible amount of air into his body, but this time he inflates his muscular structure before distributing the air throughout his body, with emphasis on his upper half. If Luffy is stopped mid-blowing, the arm will slowly deflate.
When utilizing Gear Fourth, Luffy makes extensive usage of his Haki by coating his arms, legs and much of his torso in it to the point that they turn black from hardening. The Haki coating across his body resembles the wispy ends made from flames, giving it the appearance of tattoos, and he gains shading around the inner and outer edges of his eyes. He also physically grows taller, standing around 4 meters tall. Similar to when using Gear Second, Luffy is continuously emitting steam in this form. Combined with Luffy's rubbery composition and his compressed muscle structure, Gear Fourth both adds explosive power to Luffy's attacks and grants him enhanced defensive capabilities far beyond what he can demonstrate with or without Gear Second or Gear Third. Depending on the situation, Luffy can adapt Gear Fourth into different forms.
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