Actually TREEFITER, the entire body need to participate in the flexing and extending and that includes the upper body. The idea is to keep your head traveling along a path parallel to the overall slope. Just like the ceiling of the tunnel would represent. There seems to be a misunderstanding about how the entire body would have to move to absorb especially beyond the RoM available in just the legs. Not to mention dropping the hips below the knees, even without much load is pretty risky to your ACL's. Someone mentioned the Vertical Range Test and if you investigate it further you would see a tuck being part of that and a tuck involves the shoulders and arms moving forward to mitigate the overall stance falling aft when we flex that deeply.
Perhaps tactics need to be discussed a little more and especially the idea of not taking on the big impact of a line too directly into the next mogul crest. It is a function of the same staccato edge set and RPO release and projecting the core too directly down the hill. Rounder turns where you finish your turns more and project your core towards the apex of the next turn instead of the top of the next mogul is the key. Eventually you will return to zipper line turns if you wish but with the significant change of not slamming the edges into the next mogul after the big rotary pivot move.
As far as the hands, well disciplined hands are a hallmark of all good skiers for a reason. Same goes for a good pole grip where the pinky and ring finger are used as much if not more than the first two fingers. I still remember a comment from a past mentor of mine (Serge Couttet) who scolded me for extending my pinkies and flicking my poles rather than planting it vertically and snapping my wrist. The PG version is that is it was too effeminate and it suggested I was not moving the shoulder and body far enough into the turn for the pole to be planted vertically. Years later Katy Fry shared the same idea (without the derogatory comment) with our training groups and she mentioned how her mentor changed her pole plant as part of her demo team training. Considering her tenure on that team it was worth paying attention when she shared one of the things the team selectors looked for at the team try outs. According to her, swinging the arm forward and following it into the turn was something many did back in the day but it was replaced by keeping the hand forward and using an ulnar deviation of the wrist, then to complete the pole plant all you need to do is pronate the hand. "Never show the back of your hand to the bottom of the hill" was part of that advice.
As far as core tension and functional upper body tension, well the strong pole grip tends to quiet down the arms and prevents the dropping hand but if the shoulder drops aft and down, or if the pelvis rotates into the turn the whole system breaks down and excessive rotary is the usual outcome. Then again WC stars reaching to block a gate can resemble this shoulder position but if you look lower the rest of their body is usually spot on. Not always though since recoveries are a big part of racing that fast. Anyways some tension quiets the arms and shoulders, too much makes them rigid and they get bounced around rather easily. It's a Goldilocks thing to find that just right zone but imagine carrying a long rather heavy pole. Too little grip and arm tension and you drop it, Too much tension and it swings around uncontrollably. The core by comparison is held with a lot more tension, just like when doing perturbation drills. A favorite of mine is the reciprocal pairs pushing on each other's shoulder. At first letting the shoulder gives way to the partner's hand pushing on it. That is pretty relaxed. Then the shoulder is held in place by resisiting the hand pushing on it. That's pretty strongly engaged. The later is closer to the amount of core tension we should carry but the ironic part of that is we really can't consciously engage the core muscles like we can the superficial abdominals. I've found the core takes care of that better if we do some perturbation training in our daily work outs.