For those who care to follow along or check me in my mechanical analyses of the foregoing videos, the physiological interpretation of postural control I have used is that of active, intermittent, cyclic exploitation of natural static instability for actively dynamic control.
My working mechanical model of natural movement and its relationship to aiki tai-sabaki and technique is along the lines of a double inverted pendulum, dual-eccentric center pivots, and a chaotic, cyclically actuated and damped balance system.
A nice summary of the state of knowledge I am relying on with regard to contending physical theories of human posture and balance control is found here (with further references cited):
The state of the art described in that summary challenges seriously the thoughts that fascial mechanical stiffness or manipulations of tensions (respectively, -- passively, the "spring" model -- actively, the "actuated spring" model) can physically explain what some claim that it does in a static equlibirum forces model. There is diminishing support and increasing experimental contradiction of the mechanical stiffness or spring model as a physical theory of postural control.
The mind is an unavoidable component of the postural system's effectiveness, so as useful training imagery that model may well do the trick. A singing coach talks about singing "from the head" and "not the chest" (exceedingly useful stuff, BTW), to enable an unconscious kinesthetic adjustment by means of conscious imagery -- but it has no connection to anything physical that is occurring.
The more I look at it, the more I find that understanding how uchi and soto variations operate in performing responsive jiyu-waza technique, fundamentally illustrate how the kokyu principles tutilized in aikido operate -- at both an isolated joint/limb scale and at a global whole-body scale. (I do not debate that there may be other uses of kokyu principles, but that is not my concern). There are responsive and complementary rotations that can be met (irimi), matched (tenkan), re-oriented (juji) then driven (either tenkan ir irimi) -- to cascade through the entire system of articulation.
The observations from uchi and soto turn variations in tai-sabaki for given techniques are directly cognate (although different in detail) to the tanemura-ha and shimamura-ha variations in sword work. These also manipulate the dual eccentricity of the hip joints, and a complementary rotation cascade in the joints. Suri-age and suri-otoshi are ikkyo variations that play on these differential and eccentric rotations with an additional joint/limb extension (the blade).
That is why I see aiki principles and their intimate relationship with the innovative nature of Japanese sword-work parting company, at the very least at the descriptive level, from the interpretations according to traditional Chinese natural philosophy.
A very successful, and commonly used image (along the lines of the model I am using) is of performing technique "as if holding the sword" It usually corrects a host of problems with a given technique in a very intuitively satsifying way for the student. It can also typcially be demonstrated -- with the sword in hand.
I can see why the image of spring (fascial) tensions and manipulations is temptingly suggestive in interpeting action from grasp-to-joint-to-tanden. There is no such temptation in interpreting the sword-work, where there is only contact and differential rotation(s). The last thing I want my blade (or my connection with it) to become is any kind of "spring."
A significant problem with the fascial tensions or "springs" model is that it seems not to be intuitively obvious or easily described. Perhaps for this very reason, it is not very easily communicated conceptually. I do not debate that the "feel" of it as promoted by its various advocates here may be very effective in translating the imagery that they use, but it also has serious conceptual problems as a physical model, as critically illustrated in the summary cited above.
The imagery and the physicality along the lines of the rotational motion and cyclic stability control model allow for a closer to one-to-one comparison of image to mechanical theory, and which is not linguistically or culturally colored. That is very much not the case in attempting to utilize Chinese natural philosophy concepts and its attendant imagery, back-translated through two other languages (Japanese an English) and the attendant (and sometimes contradictory) imagery of those two other cultures, and the frequently arcane or specialized usage (compared with ordinary Japanese usage) of terminology of the martial arts (whether in Chinese, Japanese or English).
Whether a technical rendering of the physical model is more or less complex as the Chinese natural philosophy rendering is, is quite beside the point. The simplistic fact is that the imagery model of sword-work illustrates the kokyu principles of aikido better and more clearly, in ways that students find more intuitively obvious, and which corrects critically important, rotational and orientational control problems in movement and technique. This may be, quite simply, because they map better onto the actual physical interactions in the employment of kokyu principles in aikido tai-jutsu, in the first place.
I know, I know .. it's just "relaxed jin"... that explains everything.