Some medium to large lowspeed aircraft have what is known as a 'spring/blow off servo tab'. This has a tab, as mentioned earlier, that operates in the opposite direction to the parent control surface but only comes into operation when the aerodynamic load on the main control surface is above a certain value. This means the pilot has direct contol over the surface at low speed, but at medium speed he is assisted by the servo tab. However another spring device comes into play when the aircraft is at high speed that allows the tab to blow back. This to prevent over- control of the surface at high speed and gives the pilot adequate 'feel' . The two 'springs' are usually in the form of torsion bars.