Steve and Erick-
... I have to take issue with some of the examples/ways that idea is interpreted here. To explain:
... If the "blade" looks like it is cutting the upper neck, or ... the head to the neck, who cares! The trick being illustrated is that there is a "right" alignment of forces and body parts. ... For any given technique, "right" might not be cutting uke's head. Or neck. Or anywhere in particular-- those are throws and pins, and it is about uke's balance, not about cutting a head with an imaginary sword.
Since I have been delving in these mechanics (in fairness, lots of doing my thinking out loud at greater length than most people have patience to endure ...
) let me make the connection a bit more concrete. A proper sword cut cuts because of a single mechanical principle.
There are several mechanical principles by which a body may be displaced, thrown or pinned. Among them -- in order of relative sophistication -- crushing, stretching, colliding, levering, and lastly shear. Aspects of each of them can be found in the others, but they are quite distinct in orientation, combination and application of action.
Only one of them is used in what we describe as aiki and it is the same mechanic as in the true cut -- shear. It is a spooky sideways cyclic, twisty thing because that principle scissors paper, severs makiwara, cracks whips, buckles columns, cuts men in half with severed cables on a flight deck, moves power through water, and flattens houses or lifts airplanes with nothing more than thin air.
So the act of imagination is not to find the sword in every canonical technique. It is grasping, intuitively, that relationship of shear that is to be applied consistently and innately and to distinguish between that mechanic and the others, which CAN ALSO be used (albeit less devastatingly or efficiently, IMO) -- but their use necessarily precludes or directly diminishes the application of the shear.
And I spent years doing the rowing exercise without having any idea what its real goal is or how to make it do that for me. But I can't blame the exercise-- funekogi undo works; MY funekogi undo didn't. So, maybe it all comes down to having the right teacher again?
More important, I think, the correct concrete image of what it is meant to accomplish in your body and then how that action is translated into another body -- animate or inanimate. A good teacher gives you that concrete image -- hands-on physically and/or descriptively.
I am a firm believer in concrete images as teachers -- O Sensei's Doka are very rich -- if read in that way. Funekogi is the "spirit of the demon snake" and furitama and tekubi furi are the "spirit of bees," in his terms -- coherent oscillation driving the shear through a system -- like cracking a whip, shaking out a rug -- or driving the shear point of cutting out the blade length and through the target -- you do the same in uke's body -- (and stiff or loose) it finds any
point of discontinuity that naturally wants to separate -- and then you help it separate.
The body deeply fears the destructive power of a resonant shear,(positive feedback -- like the whip-crack -- and the body refuses to volunteer for whip-duty) When presented with a credible rate of change of shear at certain
rhythms (hint hint hint
) -- even at low amplitude, it reacts -- reflexively -- and about ten times faster than you can think about it.