You have to have flow before there is back flow. Back flow is the stuff that might get sucked back into the faucet when you turn a hose off. It's weird physics, but it can happen under certain circumstances. I just had vacuum breakers installed on my outdoor faucets. Now when you turn the water off there is a slosh of water dumping out at the faucet from back pressure in the hose. Although my house is likely the only one that has them, my HOA requires them.

Thank you.