OK, guys, I had a play with this since I thought I might have used it as a way into the air-vents (which had lost their controlling vanes).
The mechanism for the flap is the same as many others; a spring, and a damping pot that works with a gear on the linear action. (K2edg, it sounds like your damping gear has become misaligned.)
The flap follows it's course because of the two radius arms mounted to the dash at mid-point, and to the flap at the lower edge. (Matrixum, you have pushed the latch to disconnect it, rather than toggling the latch, so it is still locked and will not cycle within the normal movement of the flap. This is your cure; remove these clips from the lower edge of the flap, and then pull the flap straight forward and exercise the latch once or twice until it operates as expected; then reattach the radius arms to the flap.)
The fully retracted position is determined by the rubber stops at end of travel. (Nobody cares about that position, as long as it's quiet!
The fully closed position is determined by the latch at the front end of travel; the door itself is also held in place by two little spring clips, one each side, and these hold the door properly aligned. (Cantthinkof1, this triggered my OCD, as well)
I did manage to lose the sled, and recovered it with a bit of bent wire; reassembling the bits made the flap work as expected. And, no, tinkering with it did not help replacing the air vent, and I had to go another way. Did it without removing the dash, though!