Taiji is supposed to be about relaxation. If that is the case, why is there such a heavy emphasis on low stances in Chen style? Doesn’t that contradict the requirement for relaxation?
The term “relaxation” is a poor word to express an important point. Relaxation is often associated with a form of limpness. Furthermore, tension and relaxation are usually treated as contradictory. However, there is a form of tension achieved through relaxation in taiji. Granted, it is not the form of tension we intuitively think of when we engage in other forms of exercise. Nevertheless, it is a kind of tension. When we are told to relax, what is usually meant is that we have to give up on using awkward strength to be able to express a more efficient form of strength.
This idea applies to the issue of leg strength. What I have found helpful in the past is to hold the stance as long as I can (to the point where my legs are so tired I feel I can no longer hold them) and then to focus on relaxing the muscles. By doing this, I have found that I need to adjust my stance slightly. By that, I do not mean that it will be higher, but that it will be aligned differently. By trying to relax the muscles, I usually have been able to find the proper alignment in the legs, and my low stances require much less muscular effort than they did in the past. In fact, they end up being sounder from a structural perspective.
One of the first times I understood this was last year, during a short period of full time training. After repeating the form for many hours during the day, my muscles simply could not do the work. It forced my body to find the proper alignment to relax the muscles, thereby expressing the tension that makes for better taijiquan.