Today also in centrifugal because of 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 be recieved at the centrifugal pumps suction and because the impeller rotates the fluid will be Centrifugal Vacuum Pump displaced away forward with a particular energy.
A vacuum pump has the inlet hooked up to one or both valve covers, sometimes the valley pan. It SUCKS the 
surroundings from the engine, thus reducing the atmosphere pressure build-up created by blow due to combustion gases going past the piston rings in to the pan.