The EVR regulates the vacuum to the EGR valve and is controlled by a pulse-width signal to open and close the EGR valve. As the duty cycle pulse width signal increases, the EVR increases the vacuum to the EGR which opens further and admits more gas into the inlet system.
Vacuum not directed to the EGR valve is vented to atmosphere. Even at zero duty cycle (no voltage) some vacuum will still pass the EVR, but this is not enough to overcome the spring pressure. It is not until the EVR is at 40% duty cycle that the vacuum is enough to overcome the EGR spring pressure.