Re: Article: Ikkyo, Nikyo, and Sankyo as Geometric Principles by Ross Robertson
Very interesting article.
If yonkyo is indeed rotation then it represents another degree of freedom. Degrees of freedom (at least in chemistry and physics) represent how different atoms within molecules can move, and are a major component of the concept of specific heat. Degrees of freedom may encompass your idea of different kinds of movement. I guess the analogy would be atoms = joints, and bones are molecular bonds.
I always thought Jujinage was both morphological and descriptive, since juji also means cross, and the forearms are crossed on the technique, kind of like a Juji gatame or the various juji jime.