In training, I've heard of guys using slap boxing as sort of a light-contact sparring. As a strike, an open-handed slap, especially over the ear, can be devastating. I once read an article by a guy who worked many years as a bouncer who claimed they were awesome for ending a fight in one shot, and he had perfected delivering them in such a way that his intended mark couldn't see it coming (and the effectiveness of the technique was enhanced by the humiliation most guys feel getting bitch-slapped unconscious). I believe the concussive force caused by a slightly-cupped hand over the ear disrupts the cochlea in your inner ear, which causes loss of balance and brief disorientation.
I mayself can attest to it's effectiveness; I got jumped once, a long time ago, and one of the guys caught me with an open-hand slap I hardly saw coming; I went down like a sack of potatoes.