...It would be great if others could similarly explain the core concepts in their wing chun
IMO it would take too much time to put into words what and why we do dan chi the way we do. Even if I tried, somebody from another lineage would probably misunderstand me. On the other hand it is very easy to
demonstrate the differences in person. Still I'll give it a go if I must.
In brief, our dan chi focuses on developing springy energy extending forward along centerline, and sensitivity to any wavering of our opponent's springy energy or deviation from centerline which we can exploit. We do train for
lin siu di daa (simultaneous deflect and attack) but of more importance to us is springy energy and learning to let the opponent's attack compress our arm (springy energy) causing our elbow to sink and make our defense ...in short
borrowing his force. This is also key to greater efficiency and avoids clashing force.
As for the wrist dropping or arm lowering in the jum sau... this is not a jut-like motion, nor is it withdrawing force. To the contrary, we strive to maintain gentle, springy, forward pressure at all times, but we learn to allow our opponent's strong palm strike to stick and compress our jum sau forcing it down and back like a cam deflecting his attack with his own power. To the observer, it may look like the jum is dropping and withdrawing, but in fact there is always forward pressure and intent.
This dan chi has a very different feeling and training objective than dan chi using a downward jut, or using a forceful, forward wedging jum. Both those solutions may be functional, but train different attributes.
Finally, to take a broader view, I should say that LFJ has a point when he says that WSL VT is, to a degree, a
different system. Similarly, my old sifu claimed his "WT" was a
different system. HKM WC is also a distinct system, etc. But LFJ is mistaken when he asserts that all other branches are
necessarily lacking in consistency, coherency, and are not integrated systems because they do not use the same techniques WSL VT uses. That's like saying an Audi isn't a functional car because it doesn't use Mercedes parts. Each is still integrated and functional.