In what direction do anti-servo tabs move relative to the primary control surface?

Anti-servo tabs move in the same direction as the primary control surface. The primary purpose of anti-servo tabs is to increase the control surface's aerodynamic stability and reduce the risk of over-controlling. When the pilot deflects the control surface, the anti-servo tab moves along with it, providing a force against the motion. This feedback helps ensure that the pilot maintains better control of the aircraft, especially during sensitive maneuvers or when the control surface is deflected significantly.

For example, if a pilot deflects an aileron upward, the anti-servo tab also moves upward. This movement increases the aerodynamic forces acting on the control surface, which requires more effort from the pilot to deflect it further, thus contributing to stability.

In contrast to this dynamic, oppositely moving tabs, random movements, or purely vertical actions would not fulfill the functional requirements of anti-servo tabs, which are designed specifically to work in unison with the primary control surfaces to improve handling characteristics.