If both right and left vents are open, air comes in through the right and left openings. However, if the right vent is open and the left closed then all air is directed down through the floor heater. So, to get fresh air 1) open the left vent, this will let outside air into the driver's compartment, if you want more air, open the right too which will direct air to the passenger side as well. If you want heat, keep the right vent open, close the left and pull out the heater valve--you can also use the blower if you are going slow. If you also want the defroster pull that out, but only part of the air is diverted to the windshield.
The damper Fid is talking about is operated by the right vent, but opening the left vent CLOSES this damper in order to direct heat (or cold if you don't have the heater vavle pulled out) through heater on the floor.
Joefrazer, what is the door flap?