How do anti siphon valves work? I'm told that you use these to shut-off a flow, when the flow is higher than normal.

I'm googling a schematic to see how these work, but luck so far.

I've come across posts in other forums where people have used anti-siphon valves on a fuel tank, with fuel being suction fed into the engine/generator. If you have a break in this scenario, wouldn't the fuel flow just stop since you loose suction (or reduced and choke the pump)? Does an anti-siphon valve add anything in this instance?

anti siphon valves are basically check valves.  a common design is a swing check valve with a flap hinged at the top.  when water flows through the valve the flap opens.  when the flow stops the flap shuts perpendicular to the pipe wall due to a stop, which prevents the water downstream of the valve from flowing back upstream.  

i think the situation you are talking about is a set up where you have a fuel line to a tank below the level of the fuel, with a loop that goes above the tank level, back down and then to the engine.  if there is a break in the line right before the engine, the entire tank can drain out into the environment due to the siphoning effect.  if the line breaks, the anti siphon valve (which is set at the top of the loop perpendicular to the fuel line with one side open to atmosphere) will introduce atmospheric air into the fuel line and break the siphon effect so you dont drain the entire tank.  during normal operation, the anti siphon valve shuts and does not allow fuel to spray out the top of the loop.

not 100% sure if this is what you were looking for