Although Yury seems to have liked the answer here, I have to add the sitting motion that I see more often than not is derived by people weighting or unwieghting, nor is it derived from maintaining an athletic stance. In fact I find it to be balance issues more often than not. People start their turns in balance, and let the skis run through the turns, using the sitting motion as a mild hockey stop type speed control. The other way that I tend to see a sitting motion, is from people who over rotate their skis to initiate the turns and then push the heels out at the end of the turn, instead of rolling onto the edge to initiate the turn and letting the skis move away from the hip, maintaining balance throughout the turn.
In terms of racing technique, similar to the video Yury posted, sitting back at the end of the turn to load the tail and then launch the center forward (to accentuate the forward pressure into the turn) into the next turn has been around for a long time, and can be a very valuable "rescue" tactic, however, this technique can cause the skier to jump (let go of the snow) in between turns, which, in fact is slower and obviously creates a moment of lack of control (when the skis are off the ground). More modern racing technique has been focused on driving through all the turns, and maintaining a more constant forward driving of the skis through the turns.