The on board Black Tank Flush only flushes out the Black tank, not the Grey tank. So you attach a hose to the inlet and under pressure the water goes through a back flow prevention valve, then into the black tank. Because of this valve it needs to be under pressure, then inside the tank are nozzles that spray water inside the tank. This is way better than just running more water down the toilet and does a great job of flushing out the tank.
2014 Georgetown 335DS
Lincoln MKX toad