Buhujapidasana and it's role in jump back practice.

What is it with Buhujapidasana? I've heard it mentioned before in relation to the jump back. obviously there's the obvious crossed ankles, the leaning forward and drawing the feet through but I think there's something else.
I did my best jump back after Buhujapidasana this morning. maybe we're usually tired by the time we get there and feel a little weak and don't notice but today I was feeling strong all the way through my practice. Perhaps something to do with last night's Tempura.
Is it
a. the hip opening aspect?
or
b. the act of crossing the ankles and really pulling them together so tightly

Need one of the anatomy guys to tell us if there's something in the hip flexors that allows us to pull our legs in closer to our backsides, might ask this on one of the forums. If I get a response I'll add it in here later.

I'm going with b. though. See my last post, I'm convinced the last piece of the puzzle for me is in the ankles.