Over each cycle a volume is created in contact with the chamber where pressure is to be lowered.
By random motion, molecules from the vacuum chamber pass into the volume created by the pump.
This volume is then cut off from the chamber and compressed in contact with a one way valve.
The maximum efficiency of a rotary compression pump is the ratio between the volume in contact with the vacuum and the smallest contracted volume in contact with the valve.
This limitation makes rotary compression pumps very hard to construct if lower pressures than about 1 Torr is to be achieved.