When you "goose" the throttle, you are creating and putting out more exhaust. This extra exhaust is being applied to the flap and overcoming the little temp sensitive spring--just be glad it is free and working--it can be a pain if the flap sticks in the shut position-restricted exhaust flow and cooked carb- (vapor lock) conditions can be had. If stuck in the open position you only need to run the choke a little longer as it will be a little "cold-natured" 'till it gets warmed up.
In the fifties, when the old chevys were in their heyday--those flaps were usually removed when somebody had their manifolds "split" for duals (this is in Texas) and nobody cried about it. As it wasn't missed, and you always had the good sounding free flowing exhaust.
Just curious -- how old are you guys- that is, what generation are you guys from-- I'm from the flathead V8 and split manifold six hot rod era (oh yeah, and the twin valve cover V8 stuff too)
