Duty Cycle is defined as the running time vs. the resting time of the compressor motor (pump). i.e. a 50% duty cycle (which is ideal) is when the motor (pump) "runs" and rests equal amount of the total time the compressor is turned on. Air is drawn from the air  tank when the motor is "at rest". Therefore, it is extremely important to  match your air tools requirements with the compressor specifications when selecting a compressor. This will avoid overworking the compressor which will cause overheating problems and lead to compressor failure.