An anti-siphon valve is a one-way valve that prevents this. If the pressure in your sprinkler system significantly drops, water from the lawn can siphon back through the pipes and into your ...

An anti-siphon valve is a one-way valve that prevents this. It can have a rubber diaphragm that prevents backups, a spring-loaded check valve, or an air relief valve.

ANSWER: The anti-siphon valve is for safety. On many boats, if the fuel line between the engine and the fuel tank breaks, the gas could be siphoned into the bilge.