Your wrists are tied, you have "refresh" on, and you change a leg pose (e.g. from "sit" to "lie-up"). Suddenly your avie jitters or flutters between two poses. This jitter happens intermittently.

Moving (i.e. walking) will fix the jitter, till you change leg pose again. Changing a wrist pose will also fix the jitter for now. Turning "refresh off" on the cuff menu will also fix this permanently. Sometimes, the jitter would stop by itself.

This was a complex bug that was related to an SL animation glitch.

Fix:

Update your cuffs to v2.1.1 (with HUD v5.1.4).

You can use "refresh" on again without fear.