Slap and plucking together?

I have a video of a live set where the bass player is playing with some wierd technique. It looks like he's slapping with his thumb but he has to be doing more than that cause the notes he's playing are faster than his thumb is hitting the strings. I'm guessing he's plucking with his fingers as well but I can't imagine how he does it really. The problem is the video is from behind him so I can't really see. It just looks like his hands are flailing around wildly but he's playing really solidly and really fast. I guess its kinda hard to speculate if you can't see it but does anyone know what he might be doing?

 

Harley from the Cro Mags. I doubt that will help.