Turbo molecular pumps consist of moving and stator blades arranged on multiple levels. The figure below shows the relation between the angle and rotating direction of the blades. Gas molecules inbound from the high vacuum side enter the moving blades and try to exit in various directions. However, the angle and rotation of the blades cause the molecules to move toward the exhaust end and through the quickest path in the stator. Gas molecules that flow back from the rearmost stage through the stator hit the moving blade, sending them back to the exhaust end.