What is ki?
Does it only exist within the fascia? How do we know?
How does it flow?
How is ki related to "IP/IT/IS"?
What is its function?
A lot of what you see in the movies as "hard training," such as holding the horse stance, etc., is involved with stressing the muscles until you learn to rely on the fascia. And the reason they do that is so that you will be very aware of that level (though they may not tell you what you're supposed to become aware of). But a good teacher can lead you through that to experience the fascia in a direct way. And after a certain amount of that, you might start feeling the ki.
For me, that was some of what the Aunkai exercises did. But in my case, I was aware and thinking about the fascia for a while before I did a lot of those exercises. So I think a lot of the old traditional stuff was about making you feel it by giving you something you couldn't avoid feeling.
As for what is ki....from my perspective, I think it's pretty well covered in the "Ki Eureka" thread, but here's some more. Does it only exist in the fascia? Well...yes and no.
I think this is a pretty clear summary of traditional cosmology on that: first is non-existence. We are not. The door from non-existence to existence is "kokoro," or heart (or mind).
What comes through the "portal" of kokoro into "existence" is ki.
Kin cannot come into this world without form, so it emerges from kokoro as a body and this body forms a mind.
The mind is made out of the same stuff the body is, a coalescence of worldly material around this original formless ki. The body is made of ki and then the mind emerges as a part of the ki/body complex.
So do we say it only exists in the fascia? Well, it permeates the whole body...and so does fascia. The fascia connects and unifies all parts of the body, but the ki connects the mind to the body in a way that nerves cannot. I think ki provides a medium by which the whole body knows anything that touches the ki in any place: the touch is instantly known throughout the entire ki. So the whole body knows as soon as any part of the ki knows. And that's a different kind of "unifying the body."
I think the main use of the ki is to provide awareness throughout the whole body at once, making the body light and easy to coordinate, also easy to switch to heavy and hard for others to move.
My endeavors in IP/IS lead me to think that continual reduction of muscular effort allows ki to flow more easily and freely (and thus with more power). Reduction of muscular effort draws everything more and more back to the center. I say this because I have found myself consciously changing my impulses from sending effort and preparation into my arms to leaving the arms completely relaxed and free of effort. I think more of using the main body to achieve my intention than of using the arms. So my mind/nerve power/ki don't rush into my arms anymore but stay in my center.
I hope this ads some useful perspective.