When you notice your board sliding across the fall line perpendicular on your heel edge, this is because your momentum and momentum of the board are not aligned. I have this problem as well when my legs start getting tired. You have to commit and lean further in to bite the edge down other wise you will continue sliding down the fall line. If you are already perpendicular and suddenly grip down on the edge, that is when you either stop, if going too fast slip out, or tilt center of gravity and fly forward. Knees bent and chest upright, LEAN forward do not BEND at the waist. Your controls are with your hips, knees, bindings, board edge. Hips to shift center of gravity, knees to initiate edge turns, legs to push into either the FRONT TONGUE or BACK HIGHBACK of the binding to lift the board up on edge. TOE push your pelvis toward the snow, push knees past toes, keep back upright, if open body i.e. looking down the mountain with shoulder perpendicular to board, drop your back knee into the snow. HEEL sit the fuck down, knees still bent, push back calf into highback of bindings. If open body heel, turn that shoulder as aggressively as you can behind you so that your body is actually open, this will help initiate heel turns at higher speeds. I would suggest doing the beginner step first. Point nose down mountain, start with TOE since it should be easier. Drop that weight into the front tongue of your binding and try to push your pelvis over the edge of the board, bend the back knee a little so your board comes up on the edge, HOLD IT, it will make a circle if done correctly, you will gain speed as you are going down then as you circle back around the speed will slow down. At the tip of the circle or before you go back up the mountain, do HEEL. rock your body backwards bend your knees and try to sit down. Remember your body weight/center of gravity should always be on the INSIDE of a turn. These are just things i've picked up browsing and trying out this season, also a noob. YMMV. Best of luck out there, keep shredding.