Looks like he's just strumming without a pick, using his fingers lightly, them adding a little percussiveness by slapping his thumb lightly on the strings.

It's the same strumming pattern he uses for Flake, and make other people use, only he's doing the mellower version here without a pick.
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In that video, his technique is the same as when he plays better together. He hits the base note twice with his thumb, then strums up with his index finger, and then just slaps the strings to get that slap sound. when he hits the base note he hits it open first and then hits the chord base note.
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D-37K Martin
Ibenez RG123 (sucky guitar I know)
Yeah, probably a style rather than a technique. I assume that he didn't 'invent' it anyway :P

But thanks for the replies!