Ki as it is manifested in the performance of techniques is what we have when the components of correct posture, center line, breathing, the explosive power of focused energy, timing, etc., come together so that we reach the highest state of perfect balance. It might be said that "ki" is the "the mastery of balance.
Just to throw in my 2 cents and opinions. I don't trust the Shioda books as being truly, accurately representative of Shioda's thoughts. His books were, as I understand it, compiled by his students using their notes and remembrances and their takes on what they thought he said. If that's true, that would explain to me why Shioda's physical performances appeared to be on one level of movement/skills and why the books are so murky and full of people moving like robots (just a joke, folks).
Tohei's approach to ki/kokyu power is pretty good and logical, although I don't think he's as clear/explicative as he should be if he's going to start a whole style based on "ki". Shioda's use of those same basic skills shows a variation that is more practically aimed at the kokyu and aiki development side. They're just facets of the same basic jewel.
I haven't seen as much Yoshinkan as I've seen Ki-Society, but it would be interesting to watch the Yosh guys get a little less rigid and a little more ki.