(14 of 25) Yang Tai Chi 24 Form, Instructional Series: Kick With the Heel - Right 

Hi Karl, I always say that the best thing to do while training is to listen to your own body. If a stance is putting a lot of stress on your knee, it's best to listen to your body and possibly not do the stance so low until you have worked up to it a little more. However, one excellent way to think about your weight distribution on your feet is to keep it evenly dispersed across the bottom of your whole foot. Generally if your weight is mostly toward the ball of the foot you will have a tendency to lean forward and it's often easier to pull someone forward when they have their weight forward like that. If your weight is too focused on your heels it can cause you to rock your weight back and make it easier to push you over backwards. If you think of the force of your body driving straight down the center of your tibia and continuing straight into the ground from there, it often helps you regulate your weight distribution without too much forward or too much backward without having to think about it so much. However, if I were in your shoes, I would try doing the stances that you are finding problematic in a higher stance and work out your weight distribution and balance there until you're comfortable, then lower the stance a little and work on the same things until you're comfortable again, and continue to do that until you are safely and stably in a low stance. I hope that helps. I wish you all the best in your training.

Thank you, Shifu Plitt!