my description is for off jumps, not knolls or knuckles. probs one of the most fun tricks, and good for learning corks/ getting used to being slightly upside down.
what i do is come in sitting down, butt over heel peice. as you get to the jump let then hand of the side your spinning to drop back, dragging along the snow. upon reaching the lip throw the opposite arm over you body, reaching around to meet the other hand. this is going to feel weird. essentailly what your doing is using your head/shoulders to initiate the spin, but the low center of gravity is going to put you into an off axis spin. it looks really cool and you can do anywhere from 1 to 7 (as far as ive gotten). you may end up slight in the back seat (on 3's, by far the easiest) but after some time you'll get the hang of it. i might be able to put up a video tomorrow, weather permitting.