The ability to relax and sink, not simply go limp, but to get rid of all unnecessary tension without degrading structure, is very much a difficult and hard learned skill. Grandmaster Wu Jianquan (who took Wu Taiji south to Shanhai) once admitted that it took him twelve years to get his shoulders to sink and relax.) Hence, to learn such a skill there must be both the necessary instructions, and an accessible method of practice.
A related, and equally elusive skill is learning how to move from the “center” or the “dantien”. Ask most people to demonstrate using their hips, or moving from the center and they will, almost invariably do what the video below calls “swimming knees”, which is essentially wiggling the legs on top of the ankles. Key here is understanding that joints only articulate in one of two ways – they either flex or extend. Thus, the hip joint only flexes and extends, it does not turn laterally “from side to side”. When you sit in a chair, you flex the hip joint. And when you stand up you are extending the hip. (Anatomy is a little more complicated. For instance, there is lateral extension, and medial flexion of the hip, but for our purposes the two movements are flexion extension, which are initiated respectively by contraction of hip flexors and the hip extensors. The more you can keep the proper alignments which facilitate these two movements, the longer the joint will last.
It is similar for your knee joint. Properly aligned flexion and extension of the knee does not erode the integrity of the joint. Improper movement, what most people do when they try to move “from the center” will get you an early date with the surgeon.
The attached video, narrated by Adam Mizner, I consider to give excellent instruction on both relaxing and sinking, along with proper alignment and use of the hip not only for taiji purposes, but also for any martial art. Mizner is from a different Taiji lineage and was a student of Huang Sheng Shyan, who was himself a senior student of both Cheng Man-ch’ing and Yang Shao Hou. Eventually Huang emigrated to Malaysia where Mizner studied from him.
Mizner, although young, seems to have some ability, and as mentioned above I really like the both the exercise shown in this video, and Mizner’s ability to articulate what he is doing.
Pay particular attention to both his assistance on proper alignment, and his repeated valediction that the movement is “over, sink, and rotate”, not side to side.