How does an Anti-Siphon Valve Work?
Anti-siphon valves allow water to only flow in one direction. For irrigation purposes, it prevents water from the system from being siphoned back into the water supply line. Essentially, it stops unsafe water from entering a clean water supply such as the water that comes from your faucets or shower heads.
This can be particularly dangerous in situations where something like fertilizer injection system is being used to apply fertilizer through the sprinklers to the grass. An anti-siphon valve prevents the water currently resting in the sprinkler systems pipes from being siphoned back into the home where it could be emitted from a faucet and consumed.
Although some are much more complex, anti-siphon valves can be as simple as a spring-loaded one-way valve which opens when water pressure is applied to it in one direction, but closes and seals when water pressure is applied from the other direction. Most cars now have anti-siphon valves in-place to prevent gasoline theft.
Anti-siphon is a type of Backflow preventer and is an essential part of any commercial plumbing installation which is connected in any way to a potable water source. They are required by building codes everywhere.
Rainbird has a very nice video on how the Anti-Siphon valve works:
Irritrol Anti-Siphon Valves
Rainbird Anti-Siphon Valves
Hunter Anti-Siphon Valves