The mechanics of the rotary piston pump do not involve a rotor spinning around a central shaft in the way that most other vacuum technologies work, instead, it relies on a solid cylindrical piston moving in a circular path within the pumping chamber.  As the piston rotates through the chamber, a layer of oil between it and the chamber forms a bead sealing one section of the chamber from the other. 

Throughout the rotation, the volume at the pump inlet is increased while the slide pin is open allowing gasses to be drawn into the chamber.  After one rotation the slide pin closes and on the next rotation that volume will be compressed out of the exhaust along with the meniscus of oil.  The oil splashes off of a baffle back into the oil reservoir, while the pumped gas exits through the exhaust port. 

There are some other features that aid and support this process.  The piston is separated into three separate sections which are timed in harmony with each other to minimize vibration.  Also, the pumping chamber is surrounded by coolant which provides adequate cooling to remove the heat of compression in a wide variety of circumstances.