Plumbing is not something you want to put at the bottom of your to-do list, especially if you have need annual backflow testing and a certification.
Backflow can occur when something deeper in the system fails, like when a water main breaks and the pressure drops. Your home is the same system. So how it can happen is water pressure is maintained from the pumping station through the mains and lines to your home- then water flows smoothly and evenly out of any faucet or tap that you open. If the pressure drops back – pressure can then be created that will draw or begin to suck in contaminated undrinkable water into the drinking system. To prevent this backflow prevention there are assemblies that can be installed to fix or combat the issue.