Theres a bimetal in there that reacts to temperature. Once it gets hot, that plate turns and shuts the funky shaped hole you see. That spring just brings the plate back to its original position.
75 sounds about right, I believe that tests the resistance of the heater that heats the bimetal.
You can try applying a low amp and see what happens - Ive never done it personally but thats what would make sense to me....the flap should slowly close.