the hydraulic motor 22 to advance in the proper direction the computer also actuates the brake valve 58 to release the pressure on the calipers 44 and thus free the brake disk 40 the sensor 48 will send data to the computer 54 indicating the advancement of the top drive quill 32 when the computer 54 receives data from the sensor 48 indicating the top drive quill 32 has rotated the desired amount the computer 54 actuates the brake valve 58 to apply pressure to the calipers 44 and thus hold the brake disk 40 the computer also closes the motor actuator valve 60a which reactivates the counterbalance valve by utilizing the above process the operator may advance the top drive quill 32 a specific number of degrees in either direction with certainty the operator may also input a desired drill string oscillation amplitude ideally the drill string oscillation amplitude rotates the drill string 14 in one direction as far as possible without rotating the toolface then the drill string 14 is rotated in the opposite direction as far as possible without rotating the toolface this oscillation reduces the friction on the drill string 14 reduced friction improves drilling performance because more pressure may be applied to the bit 18 once the desired oscillation amplitude is entered through the monitor.