The footwell vents open if it is heating the car. Turn the temperature wheel all the way up; the center dash vents should close and the footwell vents should open.

The center dash vents open if the car needs cooling. I think you've already tested these...:)

The defroster vents open when, well, you push defrost...

The only one I don't understand is the diverter (I think that's the name), kind of a shortcut to the center vents which bypasses the heater core. Not sure when it opens - perhaps only the when the interior is very hot, way above the setpoint.

Is that tied to the recirculate button?