You're blind. Opposite leg provides power for the punch. I'd demonstrate physically, but you can try it on a heavy bag yourself.
Try throwing a right cross powered by your right foot, it doesn't work, it's weak.
The opposite leg is providing support for the weight transfer of the whole body in the case of a hook or a overhand
Don't lean forward my ass
https://i1353.photobucket.com/albums/q669/mightyfighter/maidana-overhand_zpsed5b329b.gif
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