Master Ren Guangyi’s signature 21 Form has to be one of the clearest examples of the Chen style knee wobble I’ve seen.
You can see the little wobble of the knee he does before he does a fajin in the form. This knee wobble has been the source of seemingly endless debate. Some lineages of Chen style do it, some don’t. Some do it but it’s very minimal, some do it and it’s quite exaggerated. It doesn’t even seem to be a village vs Beijing style thing either.
So, why do it? It’s a way to involve the knee in the generation of power. The best analogy I can think of is that it’s like retracting your fist before you punch, but instead of having to move your fist back (which has certain obvious martial disadvantages), you store power by moving the knee back instead. This obviously requires a conditioned body, where the hand to foot is all connected, so that when one part moves, all parts move. If you just took your average Joe off the street and they did a knee wobble, I don’t think it would do anything for their punching power. In fact, it would probably decrease it. But for somebody who has built that connection over time throughout the body, it has advantages.
Lots of people are convinced that the knee wobble is the route to ruin for your body, that it’s bio-mechanically inefficient and potentially injurious over time with hard training. I can see where they are coming from since you are using the knee as part of a whip-like kinetic chain for which the hinge joint is not well evolved.
But personally I’m not so convinced that, done properly as part of an overall training system, it is bad for the knee, necessarily. I think if you were holding your weight for a long time on a misaligned knee joint then that would be a bigger problem. But the quick flick of a fajin isn’t like that. There are also plenty of things athletes do all the time that I’d say are potentially much more dangerous. It also depends on the person. The knee is a particular weak point in the evolution of the human beings into an upright posture, and being severely overweight is very dangerous to your knee joints. And if I was severely overweight there is no way I’d be flicking my knee joints about like this, so it very much depends on the overall health of the person doing it.
Disclaimer! I’m not an orthopedic surgeon, or a physiotherapist, so I’d suggest getting your medical advice from somebody who is actually medically qualified.
The Tai Chi Notebook 
Apart from the knee wobble, his alignment looks very dangerous. This is worse than the “swimming knees” you see in some Chen practitioners. He’s moving his knees around while bearing weight. For people with very strong knees they can probably get away with this for a long time. But for people like me with average or weak knees, practicing with swimming knees can result in permanent injuries. In my case I’ve had to quit taichi altogether. Btw swimming knees seems to be more of a Chen Village thing. It’s not nearly as prominent in Chen Fake lineages for example.
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While I’m not familiar with the knee wobble (beyond this video), it appears to be a standing wave. A standing wave oscillates in time but the peak amplitude profile does not move in space. Basically, the wave seems to be standing in place. In this case, that peak seems to be at the knees.
Waves (standing or other wise) have energy. That energy is the basic driver in all sorts of things but the crack of a whip is good obvious example of the energy that a wave can hold. In the case of Chen’s knee wobble, if someone trains enough to release that energy in coordination with force moving from the foot up the leg, they are going to be projecting more force. Ideally you’d want to coordinate with breath and intention.
Vis-a-vis how hard it is on the knees… if the wave is not overdriving the knee (in particular the knee cap) in oscillation or in release, it shouldn’t do damage anymore than using a bullwhip damages the whip or waves moving on the pond damage the water.
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I’m not a fan of the knee wobble, but you see it, unfortunately. Ren Guangyi is a tough guy with gongfu, but sometimes bad habits happen. Collapsing knees are a problem, too, in some Chen style folks. Those are two things I’ve been working on during the past few years. Now that he is being called “Master,” who is going to correct him? Here is a short clip of Chen Yu doing fajin. You can see power going through the leg and body from the ground, and there has to be some slight movement through the knee because you’re transmitting force in a relaxed way, but it’s not wobbling. https://youtu.be/j_vcWq2GYXs
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