Master Chen often talks about having concave curve in the body. It sounds to me like he is describing a parabolic reflector. It’s a concave mirror essentially that reflects energy, that comes in, back toward a single focal point. Parabolic structure is also used to reflect a point of light into a diffuse light.
In relation to Taiji training, Master CHen is talking about correct alignment and orientation of the joints of the body, and how often when we think we are in the ‘ correct ‘ position, we are actually quite chaotically misaligned, and lack proper orientation. There is no focal point.
My understanding of what he means is that in the correct structure of the ‘ mother move ‘ of Practical Method all the joints power should be aiming to a central dot. That dot will have cumulative power similar to the idea of the collective beam of concentrated energy from a Parabolic reflector.
In workshop Master Chen also demo about breaking opponent incoming diffuse force with a tight force and vice versa: the concave shape also facilitate this.