Pressure that can cause water to backflow into the water supply when a user's wastewater system is at a higher pressure than the public system. The resistance to the fluid by the public system creates back pressure which creates backflow through sending the water in the reverse direction. As water flows through the pipe and comes into contact with constricted areas, it is unable to move as swiftly through those sections. However, since water and most other fluids are close-grained with very compact piping systems, the lower flow rate at these points creates a reversal of the flow rate to the beginning of the pipe. Furthermore, if there are constricted points present in the system then the user will no longer be able to insert any more water into it since there is simply no space. The only way it would be possible to do so would be if the system was expandable.