How is this done?

I've somehow managed to stumble upon a Blues guitarist I like, Aynsley Lister, and have been listening to his music all day now, and while I normally don't listen to or play a lot of Blues (I'm more of a 60's-80's Hard Rock kind of person...), I must admit that this guy is pretty good. I've got one question, though. In one of his songs, "Straight Talkin' Woman", he plays something that sounds really cool about two and a half minute into the song, and I think I've heard it in some ZZ Top songs as well before it. I just have no idea how exactly it's done. I've linked the video below, in hopes that someone can tell me how it's done.

http://www.youtube.com/watch?v=oSfeX-im3RQ (from 2:32 to 2:40, roughly)

Yup, Billy Gibbons is a big user of that lick. If you listen to La Grange you will hear it a lot. Heres is what you want to do.

Lets say we play it in the same key as the video you linked (G#). You slide up on the g string from the 18th fret to the 20th and then play the b string 19th fret and play around on those 2 notes back and fourth. Then, in this video, he goes to double stops. You get something like that.

e-------------------------------------------------------------
b------------19------19------19------19-----18--18----16----
g-----18/20------18------18-----18----------18--18----16----
D-------------------------------------------------------------
A-------------------------------------------------------------
E-------------------------------------------------------------

Theorically speaking hes sliding from the 4th degree of the world famous pentatonic box to the next note of the scale (the 5th degree) and then goes back and forth between that note and the 7th degree (wich gives that dominant 7 "blues sound").

Heres the pentatonic box pattern with that 5th added when he slides up (wich we could say his a note part of the next pentatonic box pattern)

16th fret

e|--X-----|-------|----------|---X-----|--------
b|--X(5th)|-------|----------|---X(7th)|--------
g|--X(3rd)|-------|---X(4th)-|---------|--X(the 5th but "from the next pattern")
D|--X-----|-------|---X(1st)-|---------|--------
A|--X-----|-------|---X------|---------|--------
E|--X-----|-------|----------|---X-----|----------

I hope I managed to be clear :) Have fun ! This is a great lick to have under your fingers.

Thanks SebastBerg. Great explanation. Seems to be a quite simple lick, yet it sounds really awesome.

I'm interested in how he gets that walking rhythm sound a la Stevie Ray Vaughan in Cold Shot.

You can also play it in a different way (more alternating), like how it's done in La Grange. Anders has a lesson on this. Check it out: it's the same lick but the picking is different (alternate picking). (Look at the guitar solo when the chord changes to C). Take care.