This is one of the most important things in high level skiing, and one of the most neglected in teaching. The movement
@markojp is describing seats the heel properly in the boot pocket, correctly distributes pressure along the sole of the foot, facilitates tipping of the ski from the foot, and sets up the appropriate knee angles through the turn. As he points out, it positions the CoM properly, but not by leaning on the cuff like so many do when trying to get forward; this locks up the legs and results in clunky turns.