I think what most people are mistakenly calling "knock-down-power" is actually the reflexive action of pulling away from extreme pain. Just as your body pulls away from a hot stove when it's burned, it lunges away from the pain of bullet tearing into it.
This would also explain why someone intoxicated or pumped up on adrenalin doesn't react as violently to being shot. They simply aren't feeling any pain.
A Glock in the hand,
is worth two in the safe.