I use this technique in quite a few acoustic songs. Unfortunately, it's not something that's easily explained. You've really got to see someone do it.

I'll try explaining it anyway, and by the way, this isn't necessarily the "right" way to do it, it's just the way I do it.

Put your index and middle finger together and place your hand above the strings (like you see bass players do) so that your two fingers are perpendicular to the strings. Then quickly thwack the strings with the flat of your two fingers just before the sound hole (or whatever it's called) and lift them so that the strings ring freely.

The trick is doing this in the middle of a song where you have to switch between strumming and slapping. It takes practice.