Marooned, try this. Put your boots on at home, buckle them up. Now try to flex them by closing your ankle. No fore/aft allowed.
zenny, I have done these exercises on snow for my own experience and for teaching, while using the limited range of motion that the ankles have you still get some movement in the other joints to facilitate that small ankle movement, hopping in place or even while moving is also started with the ankle joints, the knees and spine follow suit. Pay attention to the feet during these exercises as the planter flexion(active) and dorsi flexion(passive) are the movements that need to transfer over to the normal ski turns.
We all do it hopefully in our skiing, it is what what helps us manage pressure.
P.S. My whole point to this question was that you CAN'T flex the ankles alone without the other joints involved, so I asked the question to see the response and I got it from you. Thanks.