The shutters and dampers require air to CLOSE them, and they are normally open when air is lost. The shutterstats shut off air supply to the cylinders when the engine warms up, and the springs cause the dampers and shutters to open. The reason, as stated above, is because if you lose air pressure to them, they will open automatically, and still allow cooling (good) as opposed to staying shut and allowing overheating (bad). When you start your bus with no air in the system, you'll notice they don't close immediately, but as air pressure builds, they will gradually close. When you start the bus with air pressure already up, they'll close immediately (actually, they're already closed
BTW, there's a valve in the engine compartment that you can turn off the supply of air, which has the same effect as the shutterstats tripping and shutting off the air. That's in case you get a leak in the shutter or damper cylinder or air line, you can isolate them in an open position and continue till you can get it fixed. If they remained closed, you would burn up the engine or be stuck making repairs.