When the engine is cold, the plate covers the throat and restricts the flow of air, forcing the carburetor to take a richer air/fuel mixture.
When the engine warms up, it needs a leaner mixture---less fuel in the air.
Engine heat flowing through the choke mechanism mounted on the outside of the carburetor causes the plate to open up, letting more air come through the throat.
See the discussion Cuz I Wanna Know: What Does the Choke Do? on the Vintage Mustang Forum.
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