Simply put, this is a check valve ball. It works with standard NADS American "D" sanke couplers, as well as other sanke couplers that use the same ball. This helps to keep your beer in "check" by blocking backflow of liquid back through the valve. In some instances, people will lose the ball while cleaning the coupler. If this happens, never fear! We've got a replacement for you so you can keep that beer safely flowing!