no offense thaibox, but you're absolutely wrong. Going up on the balls of your feet when you hit something turns your body into a shock absorber, which is what you don't want. You want all the force you generated from the punch to be transferred into what you punched. By your back foot going up onto it's ball, you're making a weak link which in turn will abosorb the shock from the impact, thus making you rock slightly back upon impact. Coming up on the balls of your feet is only ok when you're moving toward the target "swiftly"......your forward momentum will make up for the loss in rigidity. But from a standstill, your back knee should explode toward the target, but your feet should remain flat, or as flat as possible.......with the back being "near" lockout just at impact. For other type of attacks it's ok to come up, but for the "power" punches, if you're trying to finish someone off or you're trying to break something, you have to make sure your body doesn't become the absorber of the impact.