I think this is a realy complex question which deserves a complex answer. I can only share from my limited knowledge a couple of my views. Sometimes a strike feels just right. Practising 500 punches and 1 just realy finishes with a CRACK! I put it down to being well grounded in a perfect stance and delivered with speed and accuracy. It took me a long time to grasp that speed is power. A bullet thrown at a wall will bounce off, but fired out of a gun will penetrate with ease.

Would like to hear more on this though. Good topic.