A PRV plenum is always at atmospheric pressure
There is no throttle plate. Consequently, when the intake valve closes on any of the four Venturis, that entire Venturi -- intake, throat and discharge relax to atmospheric pressure.
The path to final cylinder pressure is critical
As the piston travels toward the bottom of the cycle, the same pressure is achieved as a conventional manifold so that the same mass of air and fuel is in the cylinder at any given load at the end of the pumping cycle.
The summation of the pumping losses are considerably less with PRV because of the gradual pressure reduction ahead of the intake valve, facilitated by the contoured flow of the Venturi and expansion of the fuel. Consequently, pumping losses are much smaller at the initiation of the airflow into the cylinder and the summation of the loss over the entire piston travel is substantially reduced relative to a conventional throttle plate.
