There's a tiny little vacuum switch that sits inside the lower pan. It's temperature sensitive. When code, it routes vacuum to the two diaphragms that are (supposed to be) in the upper pan (the part that screws to the hood) and operates a baffle that opens the air path to the hose duct (RA III) or hose ducts (RA IV) that supply warm air from around the driver side exhaust manifold (RA III) and passenger side heat crossover on the intake manifold (RA IV). The routing goes: manifold vacuum source to the switch in the pan (both connections are on the underside of the pan), and from there around to the front of the pan, through a rubber grommet, and up to a that connects to the Y that feeds vacuum to both the diaphragms in the upper pan.
I'm sure there are diagrams "out there" that show the connections, I just don't happen to have one in front of me right now.