Correct me if I'm wrong, but what it looks like he's getting ready to do is push his knees under the bar while pushing his hips forward. As he stands right now, he has zero bounce left in his legs for the second pull. His legs are almost completely straight. I may not have described what I'm thinking very well--no doubt in part b/c I still don't have a completely clear picture--but it seems like where he is is what I'm thinking.
In the Romanian Deadlift, as I understand it, you keep your shins vertical, and basically sit your butt back. Obviously, you can't bring your hips in until the bar clears your knees. Of course, it uses your quads too, but there is a lot of hamstring involvement.
I don't think Coach Mac is saying to slam the bar with your hips--that would hurt--but to develop a feeling of humping the bar (that's the non-PG version). I can feel that very clearly with the kettlebell swings we do. Coach also talks about what he calls Muted Hip Function in one of the CrossFit Journals, which is basically not pushing your hips all the way forward.
I think this thrust is very compatible with an upward trend of the bar. I don't think there is any way to bend your knees without pushing your hips back. Likewise, pushing your hips in will always be accompanied by a straightening of the legs, unless you "rebend" them.
It may appear there should be a force vector forward, but the actual force is only transmitted to the bar through the arms, which go up as the legs go up, and the movement of the hips is actually secondary to the straightening of the legs, which creates an upward vector. The hips just help the legs straighten, if that makes sense.