New house/anti-siphon valve

So, I purchased a new house this winter and it's time to turn on the sprinklers...hopefully the previous owners blew them out last fall. So, I found the main valve and upon turning it on water spews out of the anti-siphon valve that sticks out of the ground about 5 feet away. I've never had an anti-siphon valve on my systems in the past so I have no idea what they really do. Is there something that I should be doing to prevent this or is it broke?

If its the vacume breaker that should stop after it reseats, on some there are valves that you open to drain the water out of the valve turn them off and it should be okay

i took it apart and found that the float was not lined up correctly so it was not seating properly. What does this anti-siphon valve do anyway?

Anti-siphons are required by code to prevent the very real possiblity that under certain circumstances dirty water from puddles in the yard, underground leaks, etc, can be siphoned back into YOUR house AND into the city main.

There are other types of backflow prevention designs, but the simplest and least expensive for homeowners is the use of antisiphon control valves, or a maste atmospheric vacuum breaker. The fines for NOT using backflow are substantial, and the dead bodies are hard to dispose of!

If you have more than one zone in the sprinkler system, you need an antisyphon valve for each zone, not just one that feeds the entire system.