There's the taiji way to go very low and the not taiji way. The not taiji way is gaining flexibility chiefly via overstretching the ligaments -- which eventually wreaks chaos with the joints, but starts out looking impressive. The taiji way is via gradually creating space inside the joints and lengthening the spine, separating each joint into the lower (yin) part and the upper (yang) part and moving in a way that maintains that space and safeguards against the cartilage grinding, synovial fluid leaking, alignments compromised, the whole structure suffering damage. That part of the taiji skill that is referred to as sung, relaxed dynamic softness, largely depends on this kind of joint use. (Every time the teacher reminds the students, "drop the shoulder," "drop the elbow," "suspend the head," etc., it's about that space you are learning to create.)
The knees are a bit different from other joints because we walk on two legs and something somewhere has to make sure we don't turn into this toy from my childhood (don't know if anyone growing up later, or elsewhere, has ever seen those -- you press the bottom where all the threads are connected and it collapses. You can get it to dance this way and that way like one of those for-show wushu practitioners... fun when you're four years old! But eventually the threads holding it together overstretch and it can't do shit anymore!)