The Hook

OK, I've been trying to learn some finger picking styles, and have been watching videos of some of my idols. I noticed a common technique. Their fingers look like unmoving nubs, and one of their fingers forms a hook. The thumb is usually used to slap the low E string. It's hard to describe, and looks rather odd. I can't finger out a way of going about learning this, or why it's helpful (looks like it would limit their playing!) What's it called?

Here's some videos, with John Mayer and Joey Eppard. Both seem to use a variant of this style.

http://www.youtube.com/watch?v=WUAhUJdmYEQ (eppard)

http://www.youtube.com/watch?v=DGqVYgriqvk (slowmo eppard)

http://www.youtube.com/watch?v=AzsEzD2fVwE (John mayer. He uses it mostly during the solo that starts around 3:10)

Thanks.