Yes, it's for your black tank. I use mine after I've drained off some of the tank to aid flushing it out. You can also wait until the tank drains completely, then flush. More than likely your manual will tell you not to use it with the valve closed, due to potential pressure build up, and the inevitable stories of people who forgot it and had the flood situation mentioned above. I really like it and would install one if it wasn't already on my fiver.