Tekkai, One Piece Wiki

Tekkai is one of the six techniques of "Rokushiki" that allows superhuman defense. Tekkai hardens the users' muscles to the level of iron, allowing them to withstand and effectively nullify any damage they might have taken from attacks. Tekkai can also be used to enhance the user's attacks by the increased density one gets from using it. The strength of Tekkai can vary for users with different physical conditions, as a person with a higher level of muscular strength would be able to take stronge

Rokushiki is based on physical strength is actually based on a mistranslation in earlier One Piece volumes : r/OnePiece

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