In other words, completely ignore the hand and lower arm, and focus on the upper arm, the humerus: rotate it inward fully, then rotate it outward fully.
Rob was correct, you can isolate the lower arm from the upper and the lower arm extends and retracts -- just incrementally less. Put your elbows on the desk (don't cheat and roll the flesh back and forth) . Still works. At a post-seminar dinner years ago Saotome was sitting by himself out of the way where hardly anyone was really watching him -- and he was just rotating his forearm in that waybakc and forth, and just looking at it. Might even mean something -- ya never know
... but how does this apply to DEVELOPING kokyu? Being able to manipulate one's biomechanical anatomy intuitively is a REPERCUSSION of understanding kokyu, i think, but just forcing body parts to move in certain ways is putting the cart before the horse, in my opinion, and might actually retard progress.
I walk with a left foot and right foot. Understanding objective mechanism AND subjective feel of using it are more useful in combination than either alone. The thing is the body parts actually WANT to move in this way. We are getting in the way of something more powerful when we isolate joints for levered action, which is a structural discontinuity precluding the free propagation of the action through the body.
These actions are not forced
-- The action propagates on its own, if you understand what NOT to do
which would impede it. Most of the control problem (there is a control problem to be solved, as with any mechanical action) involves accurately modulating this form of action in continuous action without creating levers. That's what the Aiki Taiso
(or kokyu undo
is a cyclic action of small amplitude, in some case barely perceptible, that is fed into by progressively larger or smaller structure (depending on the purpose of the action). This is a mechanism of action that provides the cyclic form (a spiral is just a 3D sine wave) of kokyu action that develops linear expansion and contraction (based on torsion tube mechanics -- tension and compression at right angles on the bias of the long axis). It is easily demonstrable and shows a consequential manner of movement that is NOT jointed leverage.
This is a sensibly "taut" form of the action, but believe it or not, the mechanics are directly
related to the "loose" actions of chains and whips (which how a pliably linked structure's "tautness" acts when released or reversed and the play in the linkages is allowed.) So this approach
encompasses both "types" of aikido movement -- "loose" and "still" -- and shows they are not fundamentally different -- though they have different uses.