I don't think I'll be able to explain it to your satisfaction with words; if I had a good CAD program with a skeletal model that'd show heat-maps for the amount of pressure applied across various bones, joints, and connective tissue I could probably get closer: holding and moving your body in a way that keeps the soft tissue deformation from strikes as shallow as possible, while minimizing skeletal deviation from a stable but agile base is the key.
The trick, of course, is that nobody's fast enough to do that kind of calculation in the middle of a fight. A simplifying model helps--the most popular model involves "chi"--but even using that model, conscious analytic thought is too slow for anything but mostly-scripted practice.
As written, I hear Mary Martin in Peter Pan saying "Clap your hands if you believe in fairies."
Instead, what about this: "You willingly suspend the disbelief being thrown up by one part of your mind so you can get something useful done in a different part."
I've spent a lot of years doing both "internal" and "external" arts.
Even though I've always 'known better', I've still been tossed on my ass more than a handful of times by a soft arts practitioner, once or twice by a guy who was almost 80 years old.
It's an interesting thing, you can get tossed around for a couple of reasons:
One, you can be a subconsciously willing partner - even if you don't realize it, you want your partner to succeed because it validates what you're spending your time doing.
And two, your partner cheats and uses force in a sly way.
Taiji, Xingyi and Bagua guys are absolute masters at body mechanics. After lots of years spent practicing, they know as much about body leverage as a lot of judo guys. It can be shocking, and your body often won't realize what's happened because it encounters an overload - there's a foot and two hands touching you all of the sudden, and your brain doesn't know what to make of the sudden stimulus when you're already in a mentally stressed state. I honestly don't think that a lot of the practitioners even realize it, hence the devotion to "chi flow".
I've spent many many years studying kinetics as it relates to power generation, and the internal artists are amazingly good at it; it's just not immediately obvious what they are doing.
> The trick, of course, is that nobody's fast enough
> to do that kind of calculation in the middle of a fight.
Indeed. And not only that, but the time that it takes to observe an incoming punch and instruct your muscles to make an appropriate reaction (even if you know exactly what you wanted to do already) is on the order of 100-250 msec. If the punch comes faster than that, you are doomed.
Fortunately, there is a simple solution to the paradox: you learn to anticipate what your opponent is going to do so you can start your response before they even initiate their movement.
If you were an inert body that could be the case, but given our ability to react to the environment there is a big difference between a punch you are expecting and one that catches you unprepared.
