I can think of a couple of explanations for your knee situation: first, if the exercise you are doing is the one I am thinking of, it is not really twisting the knee (which I would describe as taking hold of the foot and turning it clockwise or counterclockwise, without moving the upper leg), but more up and down, with some slight side to side motion (more movement is actually occuring at the hip joints, which are better designed for that sort of thing). And it is gentle, you're right about that. Another thing that may influence it is your age, or more accurately, your age when doing Tajichuan. If you were younger then, and especially depending on the movements (and so the muscles used, esp if they are held tightened for any amount of time), you could have been putting a lot of pull on a recently increased Q angle. Women, as their hips widen in puberty, increase (sometimes dramatically) the Q angle on their knees, which now are no longer aligned as in males and young girls/women. This is why teens/college female athletes have so much danger of knee injury (do I again hear kids class instructors thinking 'uh oh'). As we age we 1) get less athletic and 2) get more used to the new angles of stress being placed on our knees. But they will always be more prone to injury than a guy's because when they use their leg muscles it pulls in a straight line on the knee, and when we do it pulls at an angle.