I really have to side in with the all movements are active. All our joints are controled by two opposing muscles, agaonist and antagonist. When we talk about relaxing a joint, are'nt we really saying that we are relaxing the muscle on one side of the joint and engaging the opposing muscle on the other side of the joint? Without actively engaged muscles we're a blob on the snow. The trick as I see it is to use only what we need and relax the rest, and the least effort comes from natural alignment and body power.
Part of this failure (is this too harsh?) to recognize that there is always work being done, is the fixation in our sports culture (?) that we work and strentghten our muscles only through concentric contraction. Pumping iron. That we have to shorten our muscles under load to make them stronger. Maybe we equate leg extention or straightening of the knees with how we have been trained to think of strength training. We can should train our muscles to work in both directions, and should think of them as performing work in both directions. There is much science to back this up.
So in flexing a joint we can think of it as relaxing the muscle that made it open in the first place, but shouldn't we also think of that muscle doing "work" in the opposite direction, and the opposing muscle being engaged? There is no control in a disengaged muscle.
Can we move extention/flexion to different areas of the turn. Yes, and I have students do this in lessons when needed. Being extended (tall) in the start of the turn seems to be more natural for mid level skiers. as RickH says progresively extending into the middle of the turn allows our turns to be more dynamic. Ski short, ski tall. ski tall and short inside of one turn. Now ski tall and short and move it to a different place. Now move it back. You can do pivot slips by extending, and you can do them by retracting. and I can take this and apply it different types of terrain, turns, and conditions. It's all skiing.
Is extending and flexing pressure control? Hell yes. But in this symbiotic relationship of our body and movements we call skiing, I think it does more than just that. Don't we get ourselves into trouble when we assign a purpose to a part and a movement at the exclusion of the other effects and enhancements it has? Doesn't this border on neglect? Maybe I don't have much company in this holistic viewpoint.