Below is a response by Master Chen Zhonghua to a question about the role of the 3 joints in the "Push the Wall" exercise:
… The key is that every part of the body should move except one. In the case you have described, the one point is your rear kua. You mentioned two triangles. In reality, there are many more. You are only feeling two and how they interact at this stage. The more of these triangles there are in the body, the more stable the body structure is, especially when you need to move.
We normally hear the phrase in taiji: When one part moves, all parts move. If all parts are moving, where is the base or the fulcrum? Our centerline? I remember hearing that in push hands, we must choose a spot in our body to isolate. We then adjust all the joints in our body around that spot to equalize the incoming forces.
Additionally, the spot it not always the same joint, but changes depending upon circumstance. However in each circumstance, a point in the body must be chosen as the fixed spot. Not sure if I have the understanding correct as I'm intellectualizing right now. Intellectual understanding is very different from physical manifestation. Practice, Practice, Practice.
For more detailed information about the question, see the post: 3 joint structures in “pushing the wall” | Chen Style Taijiquan Practical Method