The "last third" of a turn can also be thought of as the time to start preparing for the next turn. By the time a skier is 2/3 of the way through a turn they usually feel in control and comfortable (at least relatively). At this point preparation for the next turn requires a commitment down the hill/fall-line which can evoke perceptions of a fall. A natural response is to lean away from the fall line into the hill. There can also be a reflexive shift back as a "brace" against the forward speed. Thus, when we observe that "in the last third of the turn the body falls back and in towards the hill" what we are observing is a skier choosing a perceived postion of safety and stability as opposed to movement towards a fall and/or increasing speed - a very natural behavior.
How does anyone overcome this reflexive behavior to a perceived fear of falling or speed? They come to perceive the associated movements as something other falling or going too fast, they perceive it as SKIING. I don't think we can ever totally eliminate the reflexive behaviors. I have had the privelege to ski in person with a few world class free skiers. At below-mach speeds over non-exposed they routes almost never display such "negative" reflexive movements. But when they go to their own extreme you can see exactly some of these same behaviors. With such skiers, however, these negative movements are corrected very quickly on the rare occasions when they occur.
Now, how you get from stage A to B is a whole other story that has certainly been addressed here previously but probably deserves its own thread.