By restraining the movement of your knee and forbidding the pointing of the forefoot, you are stopping the tilt at the subtalar joint. Bones inside your foot shift around, but that subtalar joint is not being used. If you succeed actually everting or inverting the foot, and if the fore foot is constrained from moving, the tibia will rotate and the knee will move with it.
This knee/tibia movement that accompanies tilting the foot at the subtalar joint is in the direction we want our lower legs to tilt when edging a ski. So starting with tipping the foot at the subtalar joint, down inside the boot, helps tilt the lower leg. It's a byproduct of the ankle action, and a good thing for skiers to do.
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Yes. Those diagrams have been posted a number of times.
So the question: in closed chain is there really any difference initiating the tipping from the foot or from elsewhere?
According to the experiment above I (at least) can't tip my foot without the knee needing to move. There's no free play. And I'm sure I can't rotate the knee inside without the foot tipping. It all works exactly as the diagrams.
So why is there a difference in where the tipping of the foot is initiated?
I'm neutral on this - just have yet to hear a good reason that fits with my own on-snow experience.
Yes. When you 'tip' the foot/feet in the boot, it ISN'T slopping around in a loose boot, it's applying pressure in the boot through small movements in the subtalar joint. This pressure (again, not big, sloppy macro movements) tips the boot, which tips the ski. Can you put a ski on edge with bigger movements of the knee, hip, or even inclining the shoulders? Sure. But those big movements, when they originate higher up the chain, are gross movements that are very easy to compromise lateral balance. Good skiing starts from the feet. Ask any technically strong, accurate skier, and they'll confirm this. I can't think of any exceptions.
Is this guy an exception? Maybe I'm misunderstanding what he's saying, especially the bit at 4:30.