John Kells was teaching in the basement "mirror room" (had lots of mirrors).
Suddenly he went really low in the posture "Embrace Tiger, Return to Mountain". I mean really low!
Most of the time we were in a much higher stance. He explained to me that in order to develop my tai chi I needed to sink lower.
This should not be forced, however. I had one student who thought he was being more martial by taking a lower stance but his problem is that he was using force and remained stiff.
In order to sink lower we must first relax. As the hips and sacral area open/release and the legs strengthen the body can gradually go lower. This process takes time and cannot be forced.