Above the traditional Head and Body group, I have an extra Frontal group tagged as a Head and with Face behavior. This is to control the body with that Face behavior and to allow for more head tilt*. But this extra Head tag confused Body Tracking and produced a pronounced judder effect when my puppet moved--looked like she had the caffeine shakes. This was with only Body Tracker and Limb IK armed.
Unarming the extra Face behavior did not fix it. Neither did unchecking Body Tracker's tracked handle of Head--this only nailed the Frontal Head to the background. What fixed it was temporarily removing the Head tag from Frontal. [Other extra Head tags below the actual head group didn't cause issues.]
Just in an attempt to clarify, when I say the Frontal Head was nailed to the background, I mean the body was nailed down. I guess this is because Body Tracker treats as the head the first thing it finds tagged as such, so it immobilized the body because, in this structure, it's the first thing tagged as a Head.
I later had an issue of the body not moving freely unless Body Tracker was armed. [The puppet and Standing angle are attached as Free.] I don't know if it's related, but it was right after moving an arm behind the body. I removed and reapplied the Body behavior from the top level of the puppet, and everythign was fine. So if all else fails, if things go wonky, maybe remove and reapply the Body behavior. That is if it's not just me having issues. If "it's me and not you," lemme know.