someone said that GC only plants one of his heels down, and then adarq you said it looks like a half squat position; I think I can explain that with the epiphany I made a while ago: that two footed RVJ is easier to think of as a single legged motion, in which both legs are involved differently, using different muscle groups, rather than a double legged motion - in MOST cases. Some people do jump using the same exact patterns of motor recruitment in both legs (powerlifters), but dunkers jump more, for lack of a better word, unsymmetrically, unidirectionally, TDUB is dominant on the right leg, GOlden child on the left.
The plant leg, GC's left leg, absorbs a great deal of force in da pchain to start of the motion, so you land on your heel then the second leg comes in turned inwards almost (depends on your tendency to pronate, or flatfootedness, bowed-leggedness..) and the achilles + quads, + inner thighs bring the rest of the body up, while the first leg then pushes off with quads as well and achilles.
Realize though how I said the inner thighs; they are literally pushing the body up and forward during the RVJ of only ONE leg, the 'non-dominant' leg as I like to think about it.
Therefore, my favorite lift, of which I will have a video of shortly (@theAktor) is a deadlift with the barbell in b/w the legs.
What do you think adarqui? and others? Main point - one leg is pchain dominant in the 2ft RVJ, other is quad/achilles.
And to respond to why GC's back is bent - helps him absorb more force in the pchain, it's the SSC to the RVJ, abdominals are workin it, that's what I think
here is a great vid demonstrating that -
http://www.youtube.com/watch?v=I23FCaCsxkA&playnext_from=TL&videos=yHsaxZ0_c2c