all of this confusion could be condensed into a single thread entitled 'how to recognize internal skills when you see them'
There are lots of common phenomena-based tests I've found while reading on this on the interwebs.
I like the list posted by Mark Murray in the "internal power" thread:
1. Have someone push your chest with one hand in an attempt to push you over. Really push.
2. Then two hands as hard as he can. We're talking total 100% full force of whole body pushing you.
3. Then have him pile drive into you.
4. Then even casually.. increasing to severely- pull you and push you around while you stand there without moving your feet. Let them have your wrist and let them pull you for all their worth.
5. Place your hand on his chest. Without moving your shoulders or body in any discernable way, send them 3-6' with your hand.
I grabbed this from a Kokikai site, referring to Shuji Maruyama sensei
"You push on Sensei, and it feels like pushing against a huge boulder covered with foam rubber - soft, yet immovable."
I remember reading a post where someone described pushing on Mike Sigman as "pouring energy into a hole": I believe much the same type of feeling as the above, since Mike describes it as "redirecting the pushing force into the ground".
And of course we have the well-known Ki-society ki-tests.
It's quite easy to understand the commonalities even if one is not a practitioner - I knew nothing about ki (beyond my own old misconceptions about some kind of mystical "life force") before reading about this stuff for a few weeks.
Physiologically I believe the nutshell consensus from those who have tried to explain it plainly is, "ki/kokyu/jin" is a form of body sensitivity and coordination that allows accurate, efficient force redirection through the whole human body.
I believe the bigger problem that the high-level sensei are trying to get at is how to teach this incredibly unintuitive
skill more effectively. That's a tough problem for far, far wiser heads than mine.