Basically it's using your thumb as a pick. Keep your thumb rigid and strike the string so your thumb hits the string and lands on the next string down and then bring your thumb back up and let it hit the string again as you bring your thumb back up.
I have to disagree with Thunder_fingers as imo it can sound great and can be used for many situations. I use it instead of a pick and it gives me the ability to play with pick speed but also if when it is needed i can add plucks in to give a third note.
If you are going to listen to wooten for Double thumping then I will say that his use of DTing is not for everyone. With Wooten, he's all about getting the most out as little movement as possible. He is a very experimental musician when it comes to his performances and he will try to get as many notes out of one hand motion by using little tricks.
His most famous one would be the open hammer pluck.. i think he calls it. Thats basically where he hits the open string on the first hit, hamerons the note he's going to play and then plays again when he brings his hand up.
The result is 3 notes from one hand motion. Then theres the stuff that gets real fast where he will do the open hammer pluck and then he will use his index and middle fingers to pluck the string one after each other and often all on the same string. So he in effect gets 5 notes from that one hand motion. But yea that stuff is extremely difficult
Went a bit off on one there but hope this explains it a little.
