My friend Kurt (same one I went haunted house hunting with last year) arrived to help help us movie. He had just finished a class in checking back-flow prevention valves, which is actually more interesting than it sounds.
Kurt is currently a high school English teacher who is burned out—long hours a poor pay have taken their toll on him, so he's getting into the family business of plumbing. Only it's not plumbing as in plumbing repair, it's more like plumbing engineering.
Between hauling boxes and furniture up to the apartment he would stop and explain how back-flow prevention valves work. Basically there are three chambers. The first one is at a high pressure, say, 40 PSI. It hits a plate separating the first and second chambers and that plate has a spring holding it in place, but the spring has a much less pressure, say, 5 PSI. That means the water from the first chamber enters the second chamber but at a reduced pressure, which is the difference between the water presure of the first chamber minus the pressure of the plate; in this example, the water in the second chamber is now 35 PSI. There's another plate between the second and third chambers, and this one supplies even less pressure, say 1 PSI, so the water in the final chamber will only be 34 PSI.
Now, in the bottom of the second chamber is a diaphram, and water from the first chamber is directly allowed to fill the area below the diaphram. This pushes the diaphram up, because the pressure above is lower than below (35 PSI above, 40 below). Now, if the presure in the third chamber rises, it shuts the plate, causing the pressure to rise in the second chamber. This pushes the diaphram down, opening a valve to release the water directly out of the system instead of letting it build up into the system.
Quite interesting. And you have to be careful in checking some of these back-flow valves, especially the larger ones, as they can quite literally explode in your face if you're not careful.