When you hear stable upper body and rotational stability there is really no such thing, except for the observer. To create this stable upper body, meaning one that doesn't rotate with the skis all the time, you have to actually strongly counter with the shoulders and hips, to the outside of the turn (Counteraction).
Hold your poles in front of you, each in one hand, holding from the middle of the pole, each pointing up. As you look ahead between the poles, imagine this forms a "picture frame".
The idea is to stand at the top of the run and find a target at the bottom of the run (a tree, a building, a lift) and look at it through this "picture frame". Frame it, as it were!
Then, as you come down turning left and right, do not let your hands rotate with the skis and keep your target well within the "picture frame".
You will feel a "coiling" of the core muscles as you struggle to keep the torso and hands facing down the hill, towards your target.
As you keep the picture frame vertical, you will also feel a crunching effort, as you keep the body vertical by angulation.
MORE and MORE. Then a little MORE.
You can gauge your success easily from how much you were able to take the skis across the hill without losing your "picture" and the "coiling" stress you feel. While you should look forward at your chosen target, do glimpse down to make sure the skis are coming a lot across the fall line, in transition.
One drawback of this drill is that it tends to focus your attention on the hands and perhaps shoulders, but good separation is at the hip - so try to remember to focus on the hip while doing this drill: move the hips so they also point down (ideally aligned with the ski pole handles).
Other good related drills are: