Okay, this is for info only, so you people don't go playing games. Do NOT do this to yourself. You can injure yourself this way or someone else, if you try it. So please don't?
I'm gonna tell you how you can drop someone, or at least stun them, with one blow, using a blackjack, or the edge of your hand. You can also paralyze/injure/kill them. So don't play. You don't point guns at people, so don't go beating them with a blackjack or sap to test this.
Now i'll stop being daddy.
Feel around the back of your head. Right where the two lower lobes of your skull sit, right at the nape of your neck. Feel that indentation? Top of your spine. Go a fingerwidth or two below that, and this is your intended area. Lots of nerves here. Good, swift strike from the edge of the hand or a blackjack to this area will stun someone. If not knock them out.
For you people that will be inattentive and try doing this to yourself, or someone else, make sure you strike lightly. A soft blow is all it takes to see that this area DOES produce an effect. You'll feel a good tingle in your neck and a moment of faint disorientation.
Anyway. Please remember that this is not a game, and is to be used only in a deadly force situation. Otherwise, leave well enough alone and don't go looking for 'magic spots' to hit people.