Oh how I wish I could flex my boot even half as much as that demonstration in video #2. Are any of you actually able to do that?
Yes. But it took practice.
Bend forward at the ankles in the house, with socks only and in shorts, in front of a mirror propped against the wall. Stand sideways to the mirror. See how far you can bend forward at the ankle. Take a picture with the camera down low pointing at your ankles from the side; get your entire lower leg and foot, knee included, in the picture. Prop the camera down there with its timer on, or have someone else hold the camera and take a picture. It's important to point the camera straight at the side of the foot, not tilted down or at some other angle. You are trying to find and see your fullest possible range-of-motion for the ankle-bend. Be sure the pic includes the toes, the heel and the lower leg all the way up to the knee. Bend the farthest you can bend forward at your ankle for this picture. It doesn't matter for this purpose what you are doing with your upper body, but when you ski it does. Save that for later.
Now get into your boots and stand in front of that same mirror, do the same thing, take another picture with the camera in the exact same spot, so that your picture includes the toes, the heel, and the lower leg up to the knee. See if you can bend as far forward in the boot as you did before. You should be able to.
Compare the two pictures. Are you bending forward the same amount at the ankle? Does this press your shin up against the boot cuff? Does the boot cuff prohibit you from getting as far forward as your range-of-motion allows? If so, try again and use the weight of your upper body to help press the cuff downwards so that you can max out your range of motion. Keep taking pictures until you have matched the first images. It will probably require that you press down with your body weight on the cuff. If it doesn't, you may need cuffs that are more upright.
That's what you need to do when skiing. You keep your shin up against that cuff, and when turn forces build, your body weight presses down onto the cuff and you max out your range of motion. The boot gets flexed, not with muscle power, but with the pressure of your weight and your momentum. This doesn't work at all if you're aft, by the way.
Some people have very little range of motion at the ankle. Others have more. The point is that unless you're in very stiff boots, you should be able to max out your range of motion while skiing, as long as you don't use your muscles to keep your shins at a 90 degree angle to the skis. That's what we do when standing, so it comes naturally to new skiers to keep their shins "upright" in their boots, vertical relative to the skis, leaving that gap in the front of the boot cuff that is pointed out in the second video.
As a skier, you have to train yourself to keep your shin tilted forward while skiing, so that the gap disappears. But you don't flex the cuff with muscle power, you just keep your shin in contact with it, waiting for turn forces and your good stance to get your body weight to press into the cuff.
Keeping the shin at a forward angle, not at 90 degrees, takes muscle power. We use the tibialis anterior (go here for more info:
https://www.innerbody.com/image_musc09/skel28.html) to keep that ankle "closed" and up against the bootcuff as we ski. It's just not intuitive. A skier can make it so, by focusing on it and doing deliberate practice. Watch the racers at your mountain when they are just standing around. You'll see their shin angle is less than 90 degrees. If they can do it, we all can do it. If that shin stays there while skiing, then when the forces of a turn press the upper body's weight down, the boot cuff will get flexed and the boot will apply downward pressure onto the ski's shovel.
Why do we want the cuff to flex? To avoid shin bang and save our poor legs from that pain, and to progressively slow down the force of our upper bodies pressing down, so we don't slam pressure onto the front of the ski and cause it to freak out.
