In the practice of Yang style Tai Chi, our version anyway, we avoid shaking and moving our body too much and unnecessarily.
If you understand about resonance you would know why the habit of moving the body this way would be bad for your push hands practice.
Did you get it?
OK, try reading and watching the video under the part on Resonance on The Physics Classroom here.
Do you understand now that the habit of moving a little too much is as good as offering your body’s natural frequency that can be detected and used against you by a sensitive opponent?
If your opponent can move lesser than you then he can “hear” your movements better, merge with your harmonic movement then use it against you. This is why if you watch old masters do push hands its like they hardly move and yet in the next instant their demo partner goes flying.
If this is the case why aren’t more practitioners using it?
The answer is because everyone else is also moving too much which in this case is like two wrongs making a right when two wrongs is still wrong.
On the flip side moving too much and unnecessarily will make it difficult for us to fajing because it takes too long to align our body such that we can maximize our power in a split second. By then your opponent would have jammed your attempts to fajing.
This is why the principle exhorts us to seek motion in stillness and stillness in motion. In short, resonance is bad for you. This is the science of physics at work in Tai Chi. Again, if you follow the steps I outlined for the practice of Beginning Posture in TaijiKinesis Vol 2 you should be able to reduce your susceptibility to resonance because the steps offers training for realizing motion in stillness and stillness in motion.