Don't Let the "Balloon" Flatten

A common mistake, which I came to realise when pushing hands with my teacher, is that I let my "balloon" flatten. This "balloon" is my peng, and when I let it flatten, I mean my elbow bends more than it should, creating a corner instead of the arc/circle that is required in taiji.

Sounds hard to understand? An illustration: When I push, my arm is slightly bent to create an arc. When I hit resistance (my opponent's force resisting back), my teacher taught me to relax and then continue to push. But the mistake is in the relaxing part. What my teacher meant was to relax the kua and then continue to push using the strength of my legs. What I did was to relax my arms (lessening my peng) and allow my opponent's strength to move in, bending my arms a bit more than it should, creating a corner when it should only be slightly curved. When my arms are bent too much, I cannot push properly with my legs, and in order to straighten my arms a bit, I end up having to use muscular strength, and that is when my opponent can use my muscular strength against me.

So the lesson here is that when your opponent uses force, relax the kua, don't let your "balloon" flatten, and then continue to push with your back leg. By relaxing your kua, you draw your opponent's force out, straightening his force so that you can then use it against him.