To me that's just damage you don't see, but I still feel it and wince. If you could see what is happening inside a hyperextended joint I don't think you'd call it non-violent. The option to throw the towel in is still there, which is equivalent to tapping, in my uninformed opinion. I find kicks more violent than punches too - KO by head kick makes me wince a lot more than breaking the skin, or even factoring an orbital, from a bare knuckle punch. ...I think it is lasting damage that turns my stomach the most. Cuts heal, but repeated hyperextension and brain trauma doesn't.