I'll do my best to explain.
As you know, the avionics compartment has a number of computers, and computers, when up and running, generate heat. The heat has to stay at a certain level to ensure the safe operation of the computers. Not unlike your average PC at home.
In 320 series the cooling is handled by a fan (there can be other configurations, I'm not sure). When the fuselage is cold enough the hot air from the computers is circulated in a "tube" along the fuselage. And as the fuselage is cold, it cools the air. When the fuselage warms, the cooling isn't effective enough, so the AEVC (the computer that controls the system) opens up an inlet and outlet valve on the sides of the fuselage and blows the air right through.
Don't have system schematic on hand, but I can try and look for one tomorrow. I only have a picture of 340 but that's different from 320.