 You're recirculating the same air, adding 10% fresh because you don't want to be asphyxiated, but it takes that 90% air and just cools it more and more and more. When the car gets all the way cool, the air conditioning system can kind of relax. It's saving you fuel, it's saving energy, and it doesn't have to work so hard once it gets the car cooled down. The big tip is get rid of the superheated air right off the bat.