The slap

Here's a video showing why a punch with a closed fist is highly overrated as far as self-defense goes.

You can see that the person doing the slap here clearly knows what he's doing. First, he holds his left hand to his chin in a defensive posture - some times referred to as a "fence".

He then drops his left hand, which draws attention away from his right, and then wham...

Note also that the person who does the slap turns the man on the ground on his side. It may also (hopefully) look like he's going for help. Probably a bouncer.