One can not "knock out" himself his own fist
William Mulcahy, on insurance mathematics specialist (and hence in probability Specialist): "It is theoretically possible. But this is unlikely to happen in reality.
Let me explain. Man is very difficult, but at the same time - very simple. Humanity quickly disappear if we did not have basic reflexes that protect us from hurting ourselves. We are not talking about suicide: intelligence can win the survival instinct, but you can not beat reflexes. For example, if you lean forward and begin to fall, the fall is not putting forth the hand, it is practically impossible.
Here is an example of what I'm trying to explain. During World War II, British spies caught by the Germans, often taking a pill of cyanide and died. Their intelligence told them that it was better to die so, than to be tortured to death. However, those spies, who did not have a tablet or it was not possible to accept them, had to look for other ways to kill themselves. Some have tried to hold my breath, but here's what really happened: they could hold their breath long enough to pass out, but then again, reflexively begins to breathe. Thus to cheat these reflexes are almost impossible, and therefore virtually impossible to "knock" themselves own the fist - even if you try to do this stupid thing, it will not work. If you succeed and overcome your instinct of self-preservation, to hit yourself, to overcome the reflex that protects you from getting the pain just will not succeed. "