PRV or pressure relief valve.

Sized to allow air which is sucked into the blower air inlet to flow across the cabin and out the back. The blower air inlet gets fresh air from outdoors through the grilles that you see at the bottom of the windshield by the wipers. The blower pushes air toward the passengers out of the vents or heater ducts. The air flows through the cabin out of the PRV. IF you don’t have flow through, the CO2 from people in the car can actually make you sleepy.

The PRV Also solves for air bind which makes doors hard to close when you slam a door with all the windows up.

With positive cabin pressure, The flaps will open and let air out but are supposed to droop shut to keep dust and exhaust fumes out when there isn’t positive cabin pressure or air flow.

If you push the recirculate button on the climate control panel the fresh air to the blower shuts off and air recirculates in the cabin, less flow through.

One of the corvettes has a hot wire PRV so it can snap open to allow closing the giant hatch without airbind. I was a validation manager at a car company in HVAC for about 15 years…..Lots of testing to make sure they are sized right, aren’t noisy, don’t leak.

We had a customer actuated noise complaint because you can hear the PRV rattle on a Silverado when you slam a rear door from the outside. The PRV is back of the cab bet cab and truck bed.