[dargona]

It's more like a light bouncing, but you start with your hand cupped just off of the breast. I don't technically slap my breasts but it's a light impact followed up with a "bounce", similar to how some people push the cue-ball with their sticks some time in billiards (obviously a lot softer though ^^).

Also: The idea is to use the opposite hand as your breast, so cup right hand and slap left breast, etc.