Shear. Shear is hard to "see." When a material completely fails in compression or in tension it almost always fails first in a local shear from some isolated discontinuity or local weakness. That then cascades into linear stress failure because the material is at its elastic limit, and th shear failure is the proverbial straw. In buckling the shear is all that is necessary to cause structural failure, becasue there is no "reserve" structure. See here. http://www.nbcolympics.com/weightlif...id=206808.html That man's elbow failed in a buckling shear.

Well, the fact is that all the "forms" of aiki taiso follow the shapes of and implement 3D shear dynamics. Their spiral form follows the same shapes as wingtip vortices of aerodynmaic lift -- also a phenomenon manipulating shear forces. The Aiki Taiso are adapted to express it, and structurally, endure it, and manipulate it and the waza are set piece episodes in which they can be shown linearly for the uninitiated. They can be shown in any other number of forms, if the form and structure are BOTH correct to express them. One has to be able to "see" or feel shear first, however, which is NOT easy -- in part because our bodies take that sense from us by formulating a reflexive reaction well before making it available to conscious response.

Now we are getting somewhere. The spinal reflexes that control involuntary flexion and extension are uniquely sensitive to shear loading - because every structure is weakest in shear. Thus, by applying shear loading (nikkyo, sankyo grossly -- tekubifuri, furitama, much more subtly) one both attacks the weakest structural elements AND also triggers the body's protective mechanisms designed to shield those structures from excessive shear loading -- which can be exploited in "following the failure" if moving in concert with the natural form that such reflexive action takes long before conscious reaction can correct the situation.