|Pic from Koshinkan Aikido Website|
Its not something you can replicate just by spinning. But I think I'm beginning to see how it works.
First off, it all begins with kamae. If you can stand in kamae with your center in advance position and you can actually move from center instead of legs then you could probably do this.
Basically its about advancing your chushin as if you are moving forwards being drawn by it but not yet quite reaching the stage where you need to move your feet. As uke touches you, your imaginary kamae is touching his centerline too (your hands are by the side in this case), turn from chushin and watch uke fly. Or at least that's what we hope would happen.
Trying to stablise your posture with your legs, you will find that when uke comes, invariably their power will snag into you and in the worst case plant your weights on your feet. The give of the body also sucks any power that you could have utilised.
Conversely, toughening up your body or meeting forward with body movement when Uke comes will usually lead to a clash of power that dries up any force you could have used.
Its not muscle power, its centerline. Its not timing, its sense, its not bracing with force, its atari.
I think that's about it...
Um yes, its not the actual Gozo Shioda's signature demo... but something like it. His demo has got to start from this one I think. But until I actually find out, I'm not gonna bet on my words.