Shower vacuum breakers provide back flow prevention.  Handheld shower heads are typically connected to the municipal water supply.  Soap and cleaning chemicals in the shower have the potential for back flow of polluted or contaminated water to the municipal water supply, possibly causing a health hazard. VMS vacuum breakers for shower heads is a simple, low-cost device that should be used as a back flow preventer of possible pollutants or contaminants to the water supply.  Backward movement of water is called back flow, and it can occur either by siphoning or system back pressure.
Vacuum breakers for handheld shower heads generally are attached between the flexible shower hose and the diverter valve or drop ear elbow supply.