It sounds like he does it for electric.

He's creating a sympathetic rattle on a constant basis. This sometimes happens with a snare and a bass, where certain tones will rattle the snare drum "wires" against the snare head.

Honestly? I find that sound a bit annoying at times and just can't imagine it being useful on more than just pieces of a song.
Johnny Cash did essentially the same thing with his guitar for the snare sound. It works on guitar-style (elec/acoustic guitar or bass guitar or even mandolin and banjo) instruments.

I'v never tried it on bass, and i can't say that I see much use for it.
"I am hitting my head against the walls, but the walls are giving way." - Gustav Mahler.

Quote by AeolianWolf
absolutely what will said

Yay, my first compliment!
I actually quite like that tone, has a lot more bite to it than most double bass tones I've heard, perhaps it's because of the recording or his finger technique or something. It was far too high in the mix though (imo). And a bit too boomy.