Yeah, I wonder this too. I hear like what the above poster said, the same way as in a saddle, then I also hear relax and let your toes touch the ground....??
^ I don't agree with this, because in jumping your position is different from WP.....
But I think as long as your balanced and in the sorta right position you usually ride in.
I sit normally, except that I do not force my heels down. I just let my toes hang. When I make my feet into the normal riding position with the heels down, I create tension through my body and to me, it's better to be relaxed when riding than the worry about your heels.
I hope that made sense. =P