The servo tab is an aerodynamic lever. It is connected directly to the pilot flying controls. When he moves the controls, he deflects the servo tab only, and feels aeroforces acting upon the servo tab only. When the servo tab deflects, it creates a force moment on the control surface ( elevator, aileron, rudder) which then deflects the control surface until aerodynamic moments are in equilibrium. If there is no airspeed, deflecting the servo tab has no effect on the control surface.
The geared tab is a force reducer. This time, the control surface is directly connected to the flying control - deflecting them causes the control surfaces to move. In order to reduce control forces, a geared tab moves in opposite direction when its associated control surface is deflected. Whether there is airspeed or not, the geared tab always deflects by a mechanical linkage when the control surface is deflected.