From the SA videos it looks like the shoulders are supposed to be parallel to the board at all times. The board should obviously be parallel to the ground when you launch, and the board should be parallel to the ground when you land. So your board (and you) should rotate gradually forward through the jump. That's the way I read it, and that's what it feels like on the good jumps. I sometimes land tail-first and that feels wrong, and when I land nose first it scares the shit out of me!