From what I can tell by just listening to the song, it sounds like a hammer-on & slap combination. Instread of fingering the note with your plucking hand, slap it with your thumb and let your fretting hand hammer-on to the next note...here comes the tricky part.
After you do this, you have to be able to hold the note clearly enough to walk your way back down the scale. It's really hard to describe but I'll try to get a clip up later if I get a chance.
God Bless,
-J