Re: Defining Kokyu
Does not that "jo-trick" fall outside of basic biomechanics however (if you do believe in that sort of thing)?
According to how I have attempted to define "kokyu-ryoku" and according to what others have said thus far - the "jo trick" seems to fall outside of such understandings.
Truthfully, I do not trust that trick - as it came at a time when Osensei was being made into an icon and/or into a rallying point for a burgeoning "Aikido" identity. For better or for worse, many folks during that period took ukemi that they should not have. This is my opinion no matter how "famous" such Uke (or such students from that time period) may now be.
An interesting thing... Once I had an opportunity to talk firsthand to a student of a well-known teacher that does this trick currently. As things had turned out I had trained with this person for quite some time before I knew he had trained with this teacher and had actually been one of the uke "pushing" against the jo just before they went "flying." Later, I came across a video of this teacher doing this trick and there was my friend taking that ukemi! I was shocked. However, I was also very happy because here I was given the opportunity to ask one of these "uke" if he was really pushing or not. So I went and asked him and guess what? He dodged the question like you could not imagine. (Please do not give me that "Well maybe he felt you wouldn't understand" response. He and I had talked about many topics up to that point -- including highly abstract ones, etc. He was just plain embarrassed and the silence was totally related to a desire for not being asked any more.)
Another kind of related point -- for me at least -- is that I would like to leave space for the fact that basic waza are themselves outlets for developing kokyu-ryoku (as many have said). The flip-side of this is to note that while someone might be able to do the "unbendable arm" under its normally accompanying conditions such a person is not then primed to demonstrate kokyu-ryoku under more intense or spontaneous conditions. The same thing can of course be said for training in kihon waza, but it would seem that such "cool ki tricks" have even further to travel before they could really demonstrate a kokyu-ryoku that is of actual value (e.g. operable under spontaneous martial conditions) and/or REAL.