I've previously discussed the importance of keeping the body weight in the center of the foot for maintaining proper body alignment and balance. I was concentrating on this principle the other day while doing the form and noticed that when in the bow stance, the weight in my back foot was mostly concentrated in the heel of my foot.
After thinking on this for a while, I realized the problem arose from the stance. In the bow stance, as taught by TToPA, the back foot forms about an 80 degree angle with the front foot (other styles go with 45 degrees but the issue is the same). Since the front foot is pointing forward, it is easy to shift the weight along that line and keep it in the middle of the foot. With the back foot, the middle of the foot is off to the side so it is easy to just let the weight follow the line of the leg and end in the heel. In order to get the weight to the center of the foot, you have to open the hip of the back leg so that the back knee can also move in line with the foot (i.e. knee towards big toe) - then you can shift your weight to the center of the foot without torquing the knee.
Once I made this correction, I noticed that keeping the weight in the center of the rear foot resulted in keeping the whole foot connecting to the ground - i.e. the weight is absorbed by the whole foot not just the center. I further noticed that this same principle applies to the front foot as well. The result is that while the weight is still directed to the center of the foot (i.e. 'the bubbling well'), it can be distributed across the entire foot, which makes the whole foot the contact point and the stance more stable.
Not that I should be surprised by this - in ILC, one of the basic tenets is that of '9 solid 1 empty', which is all about keeping the entire foot connected to the ground. In Taiji, it is less actively done than in ILC but it is the same principle.