IMO it's more of a thing for show and flair. It's nothing more than what you say. Instead of doing the left hand slap underhanded, you move your arm rather quickly over the neck and slap downward with your hand. For a muted string slap less dexterity is needed but if you going to be hitting a note start very slowly until you work up the movement and precision needed. On a special note make sure you warm up and stretch before you try it. You could really hurt yourself trying this cold.
Brubaker Brute Squad #24|Tecamp Amplification Club
Geddy Lee Jazz Club #174| Black and Maple#414|Lone Wolf Club #91