Unknown air flap on Audi 100 C4

Hi,
I've been driving my car for 6 years now, and I've just discovered something new: There's a flap, held in place by a spring, located under the air vent on the driver's side of the dashboard (far left, in the curve of the dashboard).
I couldn't get the (manual) air conditioner to open. Without largely disassembling the ventilation system, I can't see anything from below either.
It's not exactly essential for survival, but maybe someone knows the purpose and how to operate this valve.