Watch starting at the 2:30 time mark. (2:30-3:16). Embedding is apparently disabled, just click the play button, then watch on Youtube.
:icon_chee
That's just one way of doing it. I've seen it taught the other way as well. Really he stressed the rotation and straight elbow more than anything though. Practically everyone I've encountered who boxes though does it with the thumb up (based on my experience).