And here is Lundi again with much better thumb work. I was asked to watch Paul Galbraith and Redmond O'Toole and I noticed that they both pretty much flex much of their thumb from the 2nd joint from the tip. I tried to do that here, it felt much better. (I also braced my wrist against the body of the guitar to ensure I couldn't use arm movement to "pluck" with the thumb. This is where vertical players have an advantage, our right arms can be entirely free from the guitar, giving you great freedom of movement, but I digress.)
This is only after about 5-10 minutes of trying this, I know I can do better. Watching this videos make me realize my thumb is rigidly locked into position, which seems really unnatural. But I just watched some snippets of Galbraith and O'Toole and they have that same curve to the thumb. I think this might be because if you don't but the backwards curve into your thumb your thumb, when moving, is parallel to the string!