You probably want to rotate the hand while impacting, not before, to add rotational motion and to break things (like bones). If you rotate it fully b...

You probably want to rotate the hand while impacting, not before, to add rotational motion and to break things (like bones). If you rotate it fully before impact, your arm will over turn the bones in your forarm and the energy coming back will cause your elbow to bend while hitting a heavy target, the end result is the energy from the punch will go out your bent elbow instead of out your knuckles.

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