In addition to what Skeetersaurus says (Eewwww), if you remove the unit and replace it with an elbow, it can and likely may siphon **** back out of the line. All it takes is for the black tank level to be higher than the spray nozzle and higher than the hose hookup. Run the flush, then turn off the hose and remove it; the water still in that flush line will drain out, pulling more from the black tank as it goes. That is the very definition of a siphon. The same thing can happen if the valve malfunctions in the flow position; that is closed to the atmosphere and open to the tank. Without an "anti-siphon" valve, **** CAN siphon back out through the hose hookup.
