This is my whole problem with not using skeletal alignment with the ground, and still use less muscle. You can't do both at the same time.
I know I haven't been keeping up with the discussions. But I am kind of shocked at how stagnant things are. Chris, this is sort of the whole basic point of the training-- there is
something else besides skeletal alignment and muscle. As much as you think people like me are wrong for thinking that, you have to face the fact that we honestly think there is something.
It's fine if we think each other are wrong, but you aren't going to get mechanistic discussion when one group thinks there is something that the other group thinks is fantasy. We know it isn't fully described scientifically yet. It's just that we don't all need to wait for that in order to train!
So regarding the "what is it" question. There is a name for this. "Kokyu ryoku." It's something that appeared to be of primary importance to O-sensei. It has to do with the tanden controlling a surrounding body that is "filled" with ki. Non-scientific words like "ki" and "fill" and "center" get used because we don't have physiological data.
It all hinges on the idea that one can control ki with the intent, ki can control force (like things like rubber bands, ropes and pulleys can), and the center can manipulate a body whose ki is under load. Oh, and of course, the ki can get stronger if it is exercised. (This can be done with or without a lot of muscular activation-- i.e. ki use can be separated from muscle use)
[edit: ps, the question of how the intent controls the "ki" is of course not physiologically resolved. Does the intent make some muscle activate and pull on connective tissue? Maybe. So is it "just muscle?" The Peter Ralston motion-abort demonstration when studied at its extreme really suggests that if muscles are the mechanism by which intent controls ki, this is very different from what happens after. The intent for reaching for a coffee cup causes different things to happen in the body than does the actual reaching which happens after. They may both involve muscle but they are 2 different processes.]