It means make sure the large dump valve from the black tank is open before you use the black tank flush. Otherwise, you can easily overfill the black tank because you are putting a lot of water in there really fast. Then the overflow will go out the roof vent, or leak out around the vent pipe connection into the black tank. Either one is very very messy and definitely unsanitary.
Sometimes people without a flush system will stick a hose down the toilet to achieve the same type of flush. In that case, if the dump valve is closed, the overflow backs up through the toilet and goes all over the RV. Now THAT is a MESS.
Also... if you are hooked up to sewer...keep your black tank dump valve CLOSED. This allows the tank to fill and you get a more effective drain when you dump. It also gives you some protection in case of a sewer backup from the RV park system, which is rare but has happened.
