There's a demo of the slap feature for VAMM2 in
HZMDemos.
Here's a snippet of it:
View attachment 469336
There's not much to tell about it, you need to configure the hand joints properly to have a higher position/rotation hold ( respectively a value above 5000 and 500 works in general ).
And a slap, beyond the funny ones I do here when she grops her cheek, should be fast and with a bit of bounce back.
The best approach when you have doubts is to try the animation you wanna do "with yourself"... slap your thigh and look your body mechanics, if you can't figure it out this way, record yourself slapping and look at it frame by frame. : D