There are two "teeth" on that damper/flapper which get dry and prevent it from moving. A little grease on them will free it up. You are wise for not trying to turn it with the plastic lever as that often breaks. Push on the damper itself, usually a finger through the opening will do it, and it will move/free up. Then grease it. If the lever is straight up and down, it channels warm air into the car cab. Flip it the other way and it will channel the air up through the defroster ducts.
