I'm fortunate to have a black water tank flush on my trailer.
To use the flush, I connect a water hose to the appropriate spigot on the trailer. The black water tank valve must be open for this process. I then turn on the water, and then water is sprayed inside the tank, thereby washing out the black water tank.
I allow the water to run for several minutes. If you have a clear elbow connection on your sewer hose, you can actually see the water being discharged into the sewer dump. Once the water is running clear, I turn the water off.
I then disconnect the water hose from the trailer. This task allows any more water to drain from the black water tank. Once I'm satisfied all water has drained from the black water tank, I close the valve.