If you know these things, you can explain them in common terms (even if you opt out of doing mathematical analyses). I gave a good solid and fairly complete answer to you yesterday on keeping the undeside heavy. Now, how about explaining the physical how-to's of O-Sensei bouncing someone back who is pushing on his chest.
The book The Secrets of Judo
has some great pages on just how unstable the human body is, and they give calculations for the approximate forces required to off-balance someone in an upright posture and someone in a more deep posture. These forces are much smaller than one might expect.
If it is the video clip I am thinking of, the above, coupled with a push, a non-live environment (a public demo), the master's own students, and an aging teacher, I'm not really sure what there is to explain??