Like I wrote previously, the 180 degree turn Tai-Sabaki in Korindo Aikido is taught on the hills for beginners, as one becomes more proficient, the turn becomes full feet and one may even turn on balls of one feet if necessary.
Try standing in a relaxed way, with your feet about shoulder's width apart. For this exercise, keep you knees locked & legs straight. If you lift your toes, so you're standing on your heels, your hips shift backwards
I don't buy this explanation - turning with straight legs is basically wrong, anything you learn from this position does not necessarily carry on for other positions.
On the other hand, I have no expectations to be able to explain our 180 degree turn, not without lots of pictures (actually - my sensei wrote a book and has made all the necessary pictures - but I can't publish them - bummer).
The basics for the 180 degree Tai-Sabaki are:
The source of the turn should be the waist.
One should turn from one Kamea to the opposite Kamea with the opposite side leading.
No steps are allowed - only turning.
Now, turning on the heels, it is easier to keep your legs bent at the knee, and your center low, it is also easier to feel the waist generating the turn.
turning on the the toes makes going low and generating the turn from the heap much harder.
Hence our way of teaching 180 turns is from the heel, not from the ball of feet.