Don’t lean back. Cautious skiers make this common mistake. Nothing will hurt you more than leaning back. Weight those tails too much and the tips will spread that much faster. You risk getting “thrown into the back seat” where you have no control. Stay up and forward on your skis. Stay in the driver’s seat.
If you want to try descending with your skis in the tracks, practice on short, moderate hills in fairly fast conditions. You have to have enough speed to feel the techniques work. Pick a downhill with definite turns in it, preferably an S-turn so you really feel the transition from one turn to the next.
Alpine skiers out on the open slope will rotate the torso opposite to the direction of the turn, to set the edges more firmly and prepare for the next turn. They are taught to face down the slope, even as the skis are traveling across it.
On cross-country skis in the track, you need to rotate your torso to keep from getting thrown out of the track. It helps to tuck slightly or completely, with your pole grips together out in front of you. Swing those pole grips toward the outside of the turn. You will feel very quickly if you have rotated too far or not enough. You will also feel how the technique gives you much more security when descending at speed.
Everyone has limits, of course. It’s good to know what they are before you hit them at 30 or 40 miles per hour on a screaming descent. Practice pulling one ski out of the track so you can do a half wedge to slow down. Then practice stepping out of the track completely, onto the flat-groomed skate lane, where you can use a full wedge or series of turns to scrub off speed.
The best way to control speed is not to get it in the first place. Skiers going down a hill are supposed to have the right of way over skiers coming up, but if you come ripping around a curve and ambush someone spread out across the trail in a herringbone, it doesn’t matter who was in the right. You are also supposed to ski in control. Watch out for other trail users.
Enjoy the gift of gravity. It’s going to be there anyway. You might as well get some use out of it.