Right now also in centrifugal due to high rpm as it is directly coupled to engine the pump creates partial vacuum which means the liquid will flow through vacuum hence the liquid will become recieved in the centrifugal pumps suction and as the impeller rotates the fluid will be displaced aside forward with a particular energy.

A vacuum pump gets the inlet hooked up to 1 or both valve covers, sometimes the Centrifugal Vacuum Pump valley pan. It SUCKS the air from the engine, hence reducing the air pressure build-up created by blow due to combustion gases going at night piston rings into the pan.