Now also in Centrifugal Vacuum Pump centrifugal because of high rpm since it is 
directly coupled to electric motor the pump creates partial vacuum which means the liquid can flow through vacuum therefore the liquid will be recieved at the centrifugal pumps suction and as the impeller rotates the liquid will be displaced away forward with a specific 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, thus reducing the air flow pressure build-up created by blow due to combustion gases going past the piston rings in to the pan.