Max, from my experience, I think a good segment of the skiing public are flex and extend challenged. Myself included. This is something I not only work on myself, but something I include in just about every lesson I teach.
In Psia, we call this group of movement skills "Pressure control". Personaly I find this a little lacking even though we do use these two (flexion and extention) most to do just that, we also use them to stay balanced in fore an aft plane. Maintaining our posture over our skis from front to back is so important, and requires apropriate, functional flexion and extension.
The other overlooked benefit of good continuous, functional flexion and extention, is the ability to apropiately belnd in the other skillls as needed. Just think about what the opposite of flexion and extension movements is,,,Park and ride. If we are not flexing and extending we are holding a position, and I don't know any good teacher or skier that would argue that this is a foundation to good skiiing.
One thing I look for when assessing a skier is how the lower joints (ankles knees and hips) are working together. Whether they have equal "activity" (not range of motion) or whether there is one or two that are flexing and extending more that another. The one joint that seems to be active the least in many skiers is the ankle. This is one place where the blending becomes very obvious to me. It seems much easier and more effective to use tipping movements of the foot when we also have functional ankle felxing and extending going on, keeping us balanced over our skis and actively finessing the pressure under the skis.
The next one that I find doesn't move, flex and extend, enough in an integrated and functional way are the hips. Max if you feel that you are too hunched, then maybe you could try increasing your ankle activity. Using this lower joint activity to stay balanced could help alleviate overuse in higher joints, hips. Just a thought. Personaly I don't see visiting being hunched a negative unless you live there and it is coming from a lack of activity elsewhere. Rounding the back too much of the time may a symptom of a lack of activity elsewhere too. Using the shoulders to stay balanced instead of equal activity in lower our joints.
As far as dynamic skiing goes, when I find myself not quite managing the pressure at the end of the turn effectively or not harnessing the forces effectively for my own use, it is usually because I have not been moving continuosly enough or not using my full range of motion effectively.
Flexing and extending, long and short, very important in my book. Too me, more activity usually equates to less work. Later, RicB.