The vacuum breaker is used to avoid backflow or negative pressure in a plumbing system. It allows air to enter the water system if a siphon attempts to form. Inside the elbow is a poppet valve that is held "up" by the water pressure found in the system, closing the air entrance to the device. If the pressure in the "upstream side" is reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system, breaking the siphon. It is often required by municipal water codes and allows for quick and trouble-free draining.