Why do the eyebrows twitch on my mocap character?

Setting the neutral face in the Calibrate Facial Capture tool is a necessary step; While other calibrations may be adjusted according to your requirements. If you are tracking from a video file and a proper frame can't be spotted for referencing, then skip the calibration procedure for that area. In principle, AccuFACE can track basic performances while the brow calibrations are there to strengthen and individualize brow movements.

*When the neutral and expressive calibrations are nearly identical, you are likely to experience jittery motions, especially in the form of twitching eyebrows.

If an expression morph is too identical to the neutral state of the face (like the neutral and ‘Brow Drop’ expression below) then the activated areas may twitch during a recording. You can see this play out in this video: When neutral and expression calibrations are almost identical - YouTube.