Bouncing Off

When you try to push a rubber ball, first, with a little strength, you cause a small dent. To make a bigger dent, you need to use even more force. All the while, for every force, there is a reaction force. Thus, there is a force from the rubber ball acting on you, even as you try to push it. Eventually, you will reach a point in which you have used as much strength as you can to cause as big a dent on the surface of the rubber ball as you can. If you try to push anymore, that is when the reaction force is actually bigger than what you can take, and you bounce off from your own force.

I guess the same principle applies in taiji, when you are pushing. If your opponent peng correctly, what is going to happen is that you will eventually reach a point in which the reaction force generated by your own force is greater than you can take and you get "bounced" back by the force. And it is your own force, not your opponent's, that actually cause you to "bounce" off.

And that also says a lot about peng. You must be able to peng like a rubber ball, absorbing your opponent's force, letting him come in until he cannot push anymore and bounces off on his own.