Balloon

My teacher always likes to describe peng as being like a balloon. Your arms should not flatten, they should be round. Yet, taking this to the extreme makes the arms rigid, something which goes against the taiji principle of relaxing.

Balloons are not balls of steel or concrete. They are filled with air, and are thus elastic. When pushed, they do deflate, but then bounce right back into shape. Thus, while the idea is not for our arms to flatten, it doesn't mean that they cannot flatten. Just like a balloon, we must allow it to "deflate" and then bounce back into shape. Otherwise, we are rigid and using brute force, a big contradiction to taiji's basic principles.