The Anti-Siphon valve is a combination product, Solenoid Valve plus Atmospheric Vacuum Breaker. An AVB is a backflow prevention product. and backflow prevention is an incredibly important aspect of irrigation systems. In the USA, it's mandated that every irrigation system have a device that prevents water that has entered the irrigation system from being sucked or pushed back into the drinking water supply pipes.
In some States, it's preferred to have a device at the beginning of the system, usually a double check valve assembly. Other States, however seem to prefer that backflow prevention happens at the valve. That's why the Anti-Siphon Valve is a popular product and used exclusively in some markets. Sometimes the product is misunderstood and installed in unapproved ways, or used on systems that already have backflow taken care of. Anti-Siphon Valves only protect against Back-Siphonage, which is suction or a vacuum in the supply pipe that would suck water backward from the irrigation system. That could potentially pull fertilizer, herbicides, or even dog feces (from a puddle of water around a sprinkler) into the main water
system. The ASV does not, however protect against Backpressure, which is water being pushed backward from the irrigation system into the supply system. For that reason, never Backpressure the zone pipes by attaching a garden hose or anything similar. Sometimes, when zone pipes get contaminated by a broken pipe, it's necessary to attach a garden hose to the furthest head, remove the closest head and
push the contaminants out that way. Don't do it with an ASV, you could push dirt or rocks back into the main line. The Anti-Siphon Valve should be mounted at least 6” higher that the highest outlet or sprinkler on that zone. Never install an ASV underground, either in a valve box or direct buried. There should be no downstream shut-off valves.