[Yoda]

It seems to me that HK uses "motionless head" in reference to the snares of Bobbing and Swaying and, in that regard, the model's head approaches the ideal of "motionless" with a much tighter and acceptable tolerance.

Correct. That is the point that started this whole thread. If the Head and Upper Spine move to the right, you have Swayed. If they move up and down, you have Bobbed.

Yet the concept of a visible and unmistakeable lateral move of the upper torso is being taught in many quarters. It is geometrically incorrect and deserves the warning label Homer Kelley so justifiably applied.