Exhaust manifold flapper?

[hevymetl]

Was tracking down an exhaust leak today and noticed what looks like some kind of flapper or clapper valve at the base of the exhaust manifold. It appears to have some sort of spring or thermo control thats involved. What is this and is it needed? The spring mechanism isnt operative on mine. Should it be in a specific position for my motor to run its best? Its a 66 long bed, 2wd, c10, with a 250 I6. 4spd on floor.