TLDR: See bullet points at bottom of this post.
This video is missing a lot of context! We have to expect that these skiers already lead the turning effort with their lower joints, AND that they have enough mobility to be able to balance.
Deb appears to me to be talking about two things here: Separation, and moving with the ski (as opposed to away from the ski)
Separation
If your lower body leads the turning effort, you can (and I would suggest, should) let separation happen. Separation is when you allow those joints to keep turning without the hips and chest and shoulders turning. Separation is caused by your legs turning in the socket. It allows cool things to happen, including angulation, which allows for grip, and a very nice "coiling effect", which auto-turns your skis.
The effect of separation is a bit like a spring. If you wind up a spring and let it go, it will unwind. When you've separated, and you balance on the sweet spot of your ski, your legs will unwind into the next turn. Separation lets you ski shorter turns and steeper terrain. It also leads to angulation, which gives grip.
In the video, Deb uses a form of javelin turn to artificially creating a twist into separation. This is only to create a sensation. In normal skiing, you shouldn't have to twist into separation - it should be an outcome of turning the lower joints. I separate just by resisting the spinning forces by gently engaging my core muscles.
Moving with the ski
The more important concept I think Deb's getting at is travelling with the ski instead of moving away from it. For most skiers, if they want to go left, they'll step on the outside leg, move their hip in, and then try to balance on that outside leg. You can temporarily get a platform this way, but by tipping your body inside the turn, you're moving away from the ski. Moving away from the ski takes the pressure off the ski, which stops all turn shape from happening. The end result is a rushed, low pressure top of the arc, heavy ski at the end of the arc, and a braking, scrubbing hook back up the hill to control speed. We can do better!
If you can instead let your body travel towards the outside of the turn, THIS will build the platform, create some load on the ski, and cause the ski to bend, which lead to the change of direction. Think of it like loading a trampoline at the side of the hill. When you trampoline, you don't push away from the trampoline - you load into the trampoline in order to spring. Or think of a bow and arrow - you want the arrow to pull back against the string in order to release. In this case, your body is the arrow, and the string is the side of the hill.
Anyway, those are the two concepts I think she's working with. What it means for the skier:
- Separation helps me make strong short radius turns, and it helps me to be angulated, or balanced on the outside leg, which improves my grip.
- Separation happens when the lower joints lead the turning. This means my turns can be more powerful, more proactive, and easier to adjust (e.g. shorter vs longer) than when I use the upper body to turn.
- Separation should happen just by resisting those spinning forces with a bit of core activation - I'm not twisting my spine or pushing or pulling my hip anywhere. Too much separation creates a manufactured look, and then you're skiing for form rather than for function. And I'm sure it will block you up somehow in your skiing.
- Expert change of direction happens by letting our mass move to the outside of the arc.
- If I push away from the ski, I am no longer moving my mass to the outside of the arc; pressure will come off the ski at the top of the turn, which impedes the change of direction.