What you are calling trigger slap (not arguing with you, just never heard the term) and Evela calls milking, I just call pulling the trigger with the whole hand. It generally results in the shots going low and left (for a right hander).
Something I taught to help this was a solidly locked two hand grip. Get as much contact on the gun as you can with both the shooting hand and the support hand. If you can lock down the strong hand grip with the weak hand it tends to keep your grip consistent and still, so only the trigger finger is moving and does so straight back.
Hope this is of some help.