Isolated motion.
that's the term I was looking for. I've heard animators call it that, when one part of the body moves and the parts attached to it remain motionless.
This is an accepted practice in 2D limited animation. Fred Flinstone's head will nod and turn and talk without his body (on a different cel) moving at all, but the greater realism of our 3D models seems to make that not work for us.
I remember watching a critique where a student had a character with its hands gesturing in front of it. When the character leaned forward the hands (IK hands) remained motionless in space as if they were on an invisible hand rail. That's another example of Isolated Motion. Once you notice it, it looks very odd when it happens.