In my own thinking- turning one foot at a time( Leung shun, leung ting, ben der, Kenneth chung), stepping ad turning(Cheung), ball of your feet (K1-is a little behind the ball of the feet in a hollow)
would make me vulnerable to the rush of various kinds of grapplers. In the forms, I pivot on the central gravitational axis of my body- the coordinated turning of my knee, ankles and feet make this possible. My feet are comfortably flat on the ground. The whole foot including toe adjustment works in harmony with gravity. The heels are important corners of the triangle of a good stance.
If you stand on one foot- the ground path goes through the heel with the whole foot stabilizing the balance.The K1 point is an acupuncture point not a gravitational fixture.
Ok. Here's my thinking. Again, I'm not trying to slam anyone. I'm sure Joy has trained for and can foil the attempts of a grappler quite well. My analysis is strictly from a biomechanical perspective to look at what IMHO is the optimal way to do things.
Standing relatively upright with your weight back on your heels is practically inviting a good grappler to shoot in on you and take you down. To keep a grappler from taking you onto your back, you need to be able to quickly shift your weight forward and over the top of him as he shoots. If you are on your heels it takes just a bit more time to shift your weight forward as you try to counter the takedown. That small time lag can be critical. Compare this to standing with your weight already a bit forward at the K1 point with your knees more bent. It is also much quicker to throw your legs out to do an actual sprawl if your weight is already a bit forward near the K1 point than if you are more upright with your weight on your heels.
If someone is pressuring forward challenging your structure (whether trying to trap your arms or do a takedown, etc.) while you are pivoting, if you are pivoting on your heels then more than 50% of your weight must be back over your heels. Logic says that you are more vulnerable to go over backwards. If you are pivoting on the K1 point as someone applies pressure to your structure, then greater than 50% of your weight stays near the K1 point and you therefore have more shock absorbing capacity left in your calves and Achilles tendons. You have more ability to sink your weight forward and therefore are less likely to go over backwards.
This is all just simple biomechanics.
When you stand on one foot, most people are going to bend their knee to keep their balance. When you bend your knee, the gravitational line is going to go through your mid-foot pretty darn close to the K1 point. The only way the gravitational line is going to go through your body and down to the ground through your heel is if you are standing with your knee locked out. Just try it! Simple biomechanics!