Luffy activates it by using his legs as a pump to expand his blood vessels, which allows oxygen to get to his muscles much faster. When this happens, the blood flowing rapidly through his body creates friction, heating up his body and makes his skin glow and produce steam. This increases his attack power and overall speed, but drains his stamina and if overused makes him temporarily unable to move afterward, as well as shortening his life span.
The following tags are aliased to this tag:
gear_2
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