The back water valve has a flapper, check ball or a float-able gate inside. When water rises to the backwater valve, it either rest against a flapper with a rubber seal, causes the buoyancy of a plastic ball or a gate to rise to the back side of the upper chamber in the back water valve. This action does not let go until the sewage has subsided from the downstream side of the upper chamber.
When the back water valve is in place, the sewage cannot pass beyond the back water valve and your home is safe from sewage. However, if sewage is not coming up, keep in mind, no sewage will get out, either. In fact, one of the signals your sewer is backing up outside your home is usually the result of having water and sewage from your home, not being able to get out and the pipe becomes full with sewage. On the bright side, you have the ability to stop using water and clean up the mess, without sewage continuing to pour into your home. The next step is to find where the problem is. The blockage may be on a section of sewer pipe on your property or the city. If it is on the city sewer line, contact your local Department of Public Works.