Right now also in centrifugal because of high rpm since it is directly coupled to electric motor the pump creates partial vacuum this means the liquid will flow through vacuum therefore the liquid will be recieved in the centrifugal pumps suction and since the impeller rotates the liquid will be displaced away forward with a Centrifugal Vacuum Pump specific energy.

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