... but I still dont know what it is that is moving in the dual opposing spirals in IP. Is it my limbs, a sequence of internal muscles/physical structures contracting/activating, or is it something else?
My view, and others may certainly disagree, is that they represent stored shear stresses ("windings" some describe it)
.and dynamic pendular movement -- but the static and the dynamic are equivalently spiral as the points in the post show.
Similarly, if you look at the hand of someone with essential tremor the predominant shake is -- torsional --
What is involved in the dual spirals may equivalently and dynamically, be seen as simply driving the inherent tonic frequency with a resonance (furitama). Tekubi furi, is likewise, a driven spiral pulse out the limbs, either continuously or out to a singularity and single snap reversal.
By their nature these stresses can alternate torsionally like a torsional pendulum -- think: udefuri (at about 0:55-1:47)
That gentlemen also, FWIW, illustrates the difference that I see in the meaning of atari
as "clash" in the impact sense and the different sense as illustrated by the scissors image -- and which I see, applied in a different way, in the Endo atari
video exercise also...