Common overflow tanks have a dip tube at the top. The bottom of the dip tube reaches close to the bottom of the tank. A hose coming from the radiator overflow nipple at the side of the filler neck attaches to the dip tube.
A second pipe at the top of the overflow tank only goes down a few inches. This allows excess coolant to come out and go to the ground.
Set up this way, coolant will be sucked out of the overflow tank back into the radiator via the dip tube once the coolant in the radiator cools off and contracts causing a vacuum.
