Many years ago I had a young girl student aged 9 years, I tried an experiment by teaching her to adjust her body weight when being lifted, first she would allow herself to be lifted over six feet by the big guys, after a few moments of relaxation she could not be lifted off the mat.....no magic here
Of course there is no magic here. Unraiseable body can be taught to just about anybody. I taught it to my 5 year old daughter shortly after I learnt it myself. It is simply a matter of mind/body co-ordination, relaxation and a correct frame of mind.
I agree with the point that correct body mechanics are central to what makes aikido work, by that I mean, posture, connection to the ground through the centre and a relaxed musculature. However, the importance of the mind cannot be left out of the equation.
I think this is where there the grey area of what 'ki' is, can and can't do. There are those who pooh pooh the whole idea of 'ki' and those who seem to attribute it to everything that is. I don't know what the truth of the whole matter is. For me it provides something for my mind to work with, so if I am instructed to 'extend ki' I take it as an instruction to extend my mind beyond my body. It works for me, so I will keep doing that until something better comes along.
My teacher once asked the class if anyone could explain what 'ki' is and someone put their hand up, so he was invited to share his thoughts with the class. When he had finished, my teacher said "I'm glad you know, because I have no idea what it is...(and he added with a smile and a wink) but I know how to use it!"
Ki cannot be disproven and at the moment we have no way of proving it either, so the discussions will continue...
As for the visiting 'ki' aikido person who couldn't deliver the goods, maybe he was just not very good. I have had visitors who have come from other styles of aikido that are flummoxed by the fact that they can't throw me when they try (they are not used to non resistant following and someone remaining on balance), and the harder they try the worse it gets for them and the potentials for reversals are obvious. In other words, they are not that good either.
Although I am comfortable with the term ki as it provides a frame of reference for my not very bright mind, I have not encountered anything mystical or magical in my aikido practice. Just many things that I have not known how to explain for many years until I could replicate them.