Vent Window Operation

The lower pivot extends a few inches downward into the door, and this length of the pivot pin, in its sleeve, is what provides the resistance to movement so the vent wing can be opened and it'll remain open at the angle you set despite the force of the wind as you drive. If yours flip freely and won't stay put, the lower pivot pin is probably broken off. You'd need to find a replacement lower vent wing frame with a good pivot pin.