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.