opening the knee moves CoM Forward, closing the knee moves CoM back. Inversely, opening the ankle moves the CoM back and closing the ankle moves the CoM forward.
People seem to be a little focused on the idea that ankle and knee both have to open and close at the same time for extension. Why not close the ankle and open the knee to extend forward?
It is an error in thinking to suppose that all your joints are supposed to be equally flexed or extended. That is limited and overly simplistic thinking..there is no reason whatsoever why that needs to be the case.
But interestingly, closing the ankle has a very large influence on the position of the CoM relative to the BoS with small range of motion, while the knee on the other hand has to make a large range of motion to move the CoM the same distance forward. This is because the ankle is located next to the BoS while they knee is halfway. So which joint do you think can most easily influence fore/aft balance by positioning the CoM over the BoS (or BoS under the CoM if you prefer)?
Closing the ankle joint does.
Large knee extensions can move it forward if you are able to extend enough to do it. Close the ankle and open the knee at the same time and you are basically projecting your CoM as forward as is possible (if that is what you need to do).
Opening both the ankle and knee at the same time as in full opened extension, will basically tend to move you aft because the ankle will be much more efficient then the knee in terms of range of motion. Small ankle extensions move the CoM back a lot and require very large knee extensions to compensate forward and remain in balance. If you lag on the knee extension in any way, you'd fall aft.
On the other hand, closing your ankle just a bit more immediately puts you fore.
Some might say, "well BTS my quad muscles are so much stronger to make a knee extension then to try to move my CoM around with the ankle muscles." Ok, but remember you are on sliding surface and you can pull your feet back very easily to close the ankles and change the CoM/BoS relationship. You don't have to "heave" your CoM forward with ankle muscles. We make balancing movements on our ankles all day long by walking.
The question from the OP comes back to ball of the foot or tongue of the boot..and answer still is feel your arches, but keep your CoM from falling aft, by dynamically projecting it forward. If you go too much forward you will feel it in your BOF. If you don't go forward enough you'll feel it in your heels, but if you tend to hover around the arches, then you're doing pretty well. Some pressure on BOF during turn init is good. Some pressure on the tails during turn finish is good.
Its is NOT necessary to open the ankle in order to pressure the ball of your foot like you might briefly during turn init. All you have to do is position the CoM forward.
Try this experiment. stand up on the floor in your street shoes. Find a state of balance with relaxed stances where you are feeling your weight on your arches. Now very slightly close your ankles but change nothing else. As your CoM moves forward you will feel weight on the BOF. now slightly open you ankle...you will feel weight on your heel.
It takes only a very small ankle movement to cause your CoM to be forward enough to be standing on the BOF.. it does not require you to ride on the tongue of the boot. It does not require you to physically plantar flex like pushing on a gas pedal to develop pressure under BOF. Just position the CoM ahead of the BoS and ride into the turn, let the forces coming into the front of the ski longitudinally bend the ski. Don't linger on the BOF too long, before the apex you should already be feeling your arches.