I think it's a part on a front wheel drive car. Oh, okay, VAV= variable air volume and CV= constant volume. A constant volume box usually just has a hot water and/or chilled water coil in it with no damper in it. Not always the case though.
On edit: I see orion beat me to it. Damper, no damper, doesn't really matter if it maintains a constant flow.
Most of the time CV boxes can maintain temperature without changing the airflow to the space. This is important in areas that need special pressure gradients like operating rooms, radiological pharmacies or chemical labs.
Ya'll have explained what it is but you haven't explained WHAT it is!
What you are asking to be explained is a fan powered box that runs all of the time. Instead of the fan coming on with just the heat the fan runs all of the time. The fan is actually at the dischage of the box. That means that any air entering the box, either from the plenum or from the inlet damper, it all is being pulled through the fan. Most people call it a mixing box.
When the box is at set point the inlet damper is either closed or at minimum. So your discharge air that the fan is blowing out is basically plenum air. It's just recurculating. As you need more conditioned air the damper opens more and less plenum air is being pulled in because you are now starting to blow in conditioned air. At 100% cooling the conditioned air that is being blown into the box should be that same amount of air that you are blowing out of the fan. This is where air balance comes into play.
At 100% the box should be neutral, meaning there is no plenum are being pulled in. Its all conditioned air.
Hopefully that helps...
"If you can't explain it simply, you don't understand it well enough