Directional valves stop yard water from mixing with drinking water. In a system without a directional control, or backflow preventer valve, water from the yard could run the wrong way. The different types of backflow valves use pressure differentials to ensure a one-way flow from house to yard.
Automatic sprinklers turn on and off with electrically controlled valves. The valves control several sprinkler heads at once. When the valve turns off the system should be sealed. Faulty valves or an abnormal drop in pressure on the supply side could start backflow, sending polluted water back into the household plumbing. Control valves alone will not protect the house from backflow problems.
Both compartments of double check valves work in the same way--if one jams open, the other should still provide protection. Brass construction doesn't corrode under normal use and yearly testing reveals any problems. The two valves only open when pushed open by rising water pressure at the input side of the valve. Springs push the valves sealed when pressure drops. Without the valve in the water line, opening faucets in the house could create a siphon effect that pulls water backwards from the irrigation system. The double check valve seals the line twice when the system is off. Double check valves lie below ground level, protected from frost damage by the soil and out of the way when mowing.
The inverted U of plastic pipe and valve--called a vacuum breaker--works on a different principle. The above ground section sits at least 12 inches above the highest point of the irrigation system. For a siphon flow to begin, water must lift up and over the raised section. The breaker valve in the system includes a spring-loaded seal that opens before the suction becomes strong enough to completely lift the water. Air enters the pipe and the water drops back into the yard side of the system. As soon as the pressure stabilizes the valve seals again. When the system simply turns off, the weight of the water on the output side draws the valve open long enough to create an air gap. Under operating pressure the air bubble expels from the water line. This type of valve is exposed to damage from mowers and freezing temperatures. Removing the spring from the valve in the winter prevents frost damage. Vacuum breaker valves don't hold up to machine damage well--even a weed-eater string could shatter a plastic pipe under pressure.