A joker valve or commonly nicknamed duck valve is a replaceable rubber valve that is used in some MSD systems. They are found in manual pump flushing heads or one of the electric pump flushing types. The brand and model of the marine head is important because each manufacture uses a slightly different style and size. For example, the Raritan model is not shaped like a duckbill but instead has 3 points. The joker valve is a small one-way valve that lets water, paper and solids pass through it. Its primary purpose is to keep the flushed effluent from coming back into the toilet after flushing. With usage it wears out allowing water to backflow into the bowl after being flushed. An indication that the valve is wearing out is when the water level in the bowl rises slightly after flushing or the color of the water in the bowl goes from clear to colored. Regular use of Forget About It For Boats will not stop the joker valve from wearing out, but it will keep the resulting odor problems at bay and it will make for a much easier and cleaner job to do when it is time to replace it. Replacing it is a simple and straightforward task for the average boater. A quick google search will show you how to do it on your model.