Go find a mellow tabletop and just practice. All you do is force your wheel (rear) to slide up the face. It's kind of weird at first but once you get it a couple of times you'll understand. Just commit to it. Going half assed will get you bucked a lot of the time. But you'll find that out.what I don't understand is how he breaks traction as he's coming up the face of the jump. If the moto guys use their throttle to do that, I don't think there's any way pedalling could do anything to break traction in a mtb situation... all I can think of is some form of a prehop to lighten the bike, and then muscling the fvck out of the bike to start slipping early. but that would take wicked strength and coordination. which he seems to possess in spades.
Check some clips from the a-line races at whistler. There was one with elliot jackson from last year that's a really good example.
Gwinn isn't the only one who knows how to scrub a face on a bike btw. He might be doing the fastest right now however