Now also in centrifugal due to high rpm since it is directly coupled to motor the pump creates partial vacuum this means the liquid will certainly flow through vacuum therefore the liquid will become recieved in the centrifugal pumps Centrifugal Vacuum Pump suction and because the impeller 
rotates the liquid will be displaced aside forward with a particular energy.
A vacuum pump gets the inlet hooked up to one or both valve covers, sometimes the valley pan. It SUCKS the surroundings from the engine, therefore reducing the air pressure build up created by blow because of combustion gases going past the piston rings into the pan.