Functions of pulling the foot back
For a PMTS based view on this subject, I'll repost my response to a similar question that was originally posted in a thread on the PMTS forum - by the way, this one gets beat to death every so often over there also (http://www.realskiers.com/pmtsforum/viewtopic.php?t=489&sid=389d7a292c60cbff9b4dda91a7 957280
Pulling the foot back has a general function for balance and it also has an additional function in each part of the turn.
The general function of dorsiflexion (pulling the toes up while pushing the heel back) /Plantar flexion (pointing the toes and pushing the foot forward) is that they control fore and aft balance.
When you hear people say "get forward" or "move your balance back" and you wonder "how do I do that?" This is how. It is less important to rely on cenetered-ness "as a stance position" and more important to learn how to find cenetered as a movement. [this is completely consistent with Arcmeisters comments].
Beyond this, these movements also have other functions during the turn.
In the release phase, dorsiflexion and generally pulling the foot back helps the skis seek the fall line faster in the top of the turn even when the skis are flat -- this becomes especially important in the steeps and the bumps. This also makes sure that your body is centered at the top of the turn. Some people find that focusing on just the new free foot is the best, others find that doing the movement in both feet is more effective. Whichever works.
In the engagement phase, dorsiflexion and generally pulling the free foot back also controls fore/aft balance However, just as important, it facilitates inversion of the free foot and helps produce greater ski angles and increase engagement.
Just try standing on the slope with a naturally narrow stance. Now, push the uphill foot out in front so that the heel of the uphill foot is in front of the downhill foot. Holding this position, and without flexing your legs, try to tip the uphill foot toward its little toe edge using your foot/ankle.
With your free foot leading, you will very quickly reach the limit of your tipping ability.
Now, pull the foot back so that the toe of the uphill boot is slightly behind the toe of the downhill boot. Holding this position, try to tip the uphill foot toward its little toe edge using your foot/ankle.
You will notice a big difference in the amount of inversion that is available if you continue to invert while you hold the foot back.
Hope you find this helpful and/or interesting.