I am new to this but I want to take a stab at it so please..be gentle.
I would venture to guess that if I was able to direct the force in the front foot (based off the YouTube clip at 3:45) that you would be able to get under him more and either stop him completely or get under him to pop him back...really causing him to be off balance. I don't know if the connection to him is any different if I shift the incoming force to my back vs. my front foot. Like I said, I am new to this.
Also, in the "horse stance" example he gave at minute 3:00 where he is off balanced from a side push, shouldn't Chalandra be able to remain in that stance without loosing balance even being pushed on the side?
Now with all this talk of being "under", what exactly do people think it means to be "under" the other guy?
You could certainly be "under" from either the front foot or the rear, and could stop the pusher from either point. The front foot provides additonal mobility as you can then bring the rear foot forwards and take a step towards your opponent without "pushing" them backwards. (There is a HUGE temptation to push someone when you feel them go light/weak, but thats bad) Rather as you are under them, they will basically push themselves backwards. One might also see how this may allow them to kick an opponent as well.
If I direct more of the persons push into my front foot, it does seem to pop them more upwards like the vector I drew. Putting it in the rear foot still allows for an upwards vector, but people start to push against the floor along that line and instead of increasing pressure into the rear foot, it decreases. That's why we see Chalandra pop forwards, he isn't really doing what he says he is doing. He is pushing off the ground, instead of pushing into the ground to increase pressure in his foot. Pushing off the ground makes the supporting foot feel light, after all that how you jump off the ground! Pushing with the quads makes you go light, pushing with the kua does not.
As for the 3:00 marker, if he was underneath the other guy he would never pop off the ground. Really unless the pusher suddenly sourced power way below Chalandra, the pusher would always be pushing himself away much like if i leaned against a wall and pushed I would push myself backwards.
The rotation that Chalandra shows at 3:17 directs the push off to the side. I'm not sure if Chalandra knows how to get under someone else from this video or not. The knee up knee down thing is something else entirely. To me it looks like he is using it to brace while turning. The Chens use a completely different movement to turn. I think most people could learn a way to copy the exterior shape of the big name chens fairly easily that would not result in bracing against the push.
You of course would be better off learning the real mechanic for how the chens turn, instead of copying the shape externally, but thats beyond this disucssion.