I'll expand on Mike's interpretation a bit based on my own experience.
Folks have said that when you train the body past the point of exhaustion, you get to that "inner stone" and begin polishing it. I find this to be agreeable by my own happenings in and outside the dojo.
Take, for instance, illness, as Mike mentioned. I have been hit now with two bouts of @##@%! food poisoning in 6 months. Aside from the obvious now-abstinence of 3rd party meals
, my ki had been eaten away.
But sometimes this is when my aikido training makes the best leaps in (hopefully) understanding my aikido. I don't have the energy to stand, much less crawl, and so I'm left to my feverish thoughts.
Now I'm not saying I'm right by any means, nor am I saying that getting ill has made me a better aikidoist. Still, what I'm "learning" while sick holds a deeper understanding than I've ever had before the illness. "A new way of looking at it", you could say.
So in this respect, I find ki not be explainable, but something I learned from by not having it. When my ki (and body/mind) are replenished, I'll put the "new angle" to the test and see if it cuts the mustard.
I should add that after my first bout of food poisoning, which nearly did kill me literally, my chief instructor (MikeE) would probably tell you he saw signifcant improvement in how I execute technique. Lucky, coincidence? Maybe. I like to think of this being one of my sick little ways of polishing the stone.