The one I use attaches in place of the radiator cap.

Compressed air jet siphons the air out of the cooling system.

There is a gauge and when it reaches 20 - 25 inches of vacuum a valve is closed and the jet siphon is replaced with a suction hose. Dip the hose in the coolant, open the valve, cooling system fills by itself.

This is the only way to fill a modern cooling system. Also, if the vacuum doesn't reach 20-25 or bleeds off quickly you know there is a leak.