I thought only us "Joiseyans" were skilled in the fine art of knee cap breaking.
As for dropping to the ground, that's the easy answer. When you get hit in the knee, you lose your balance. Since the motion of your knee is important for keeping yourself stable while standing, the right strike to the knee prevents you from correcting your balance and catching yourself from falling.
As for the sick feeling, I can only guess. I'm assuming when you get hit hard enough to fall, it also hurts like heck. The sick feeling would then be the same reason you'd feel sick after any really painful injury...the rush of endogenous opioids that works as an analgesic can also make you feel a bit dizzy and/or nauseous.
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