Abstract: A system and a method for adding and subtracting stages of compression to a compressor as and when the compressor requires them. If the compressor needs only a low pressure ratio, then the system and method allow the compressor to operate with only a primary pumping circuit spinning, while available additional stages, forming a secondary pumping circuit, and which may be required at other times when the needed pressure ratios increase, are decoupled from the rotating shaft, so that the compressor pumps at its most efficient and flexible point. Further, a system and method for adding and subtracting stages of compression to a compressor in order to increase and decrease the pumping capacity as and when required to satisfy a given load requirement.