Interesting Advice, Bush. Do you get the opportunity to teach a lot of 'powder specific lessons'? Honestly, I'd love to hear about the learning curve and struggles successes you've had in that arena
I think you're right, the reason that the intermediate struggles in powder is because their main method of speed control (skidding, dragging and pivoting) gets taken away from them.
Fortunately, powder rewards good skiing and gives immediate feed back. Turn shape (or line choice) is the best way to control speed (and for those of us not on big whopping rockers, the only way!)-all you have to do is hold your turn through the fall line and you'll drop speed (usually you'll drop too much speed and sacrifice that necessary momentum for smooth powder skiing).
gary, that is a nice sequence on good powder skiing, I would add only one more element:
While most will maintain that proper flexing is essential in skiing any surface, In powder, flexing the legs to release the old turn, transfer balance and engage the next turn is even more critical (the benefits of doing it right and the consequences of doing it wrong are instantly apparent in deeper powder). Developing a nice, rhythmic flexing pattern in powder is both enjoyable and helpful in skiing it well. It is not all that dissimilar from smooth mogul skiing (which, in my book, is a closer analogy to good powder skiing than good groomer skiing; though, powder skiing is a lot easier than good mogul skiing once you get the hang of it).
The best explanation, photo sequence and tutorial on how to build good powder turns (that pays very close attention to flexing) is in Deslaurier's 'Ski The Whole Mountain' book. If you are in to PMTS you'll find Eric and Rob's approach very familiar. I can say in all honesty that book was the best instructional help I ever had regarding skiing Powder (and crud and steeps....it's less helpful for moguls). It'll give you the basic concepts and movements, after that, you'll just have to seek out the soft stuff and get some miles.
Get after it!