It is not connected to the trailers potable water system but it is connected the the source water system when used, thus the need for a vacuum break.
While it is almost impossible to have a condition where the source system would pull a vacuum on the black tank while the nozzle was under water that is what the protection is addressing.
Do the world a favor. Don't ever use the thing.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles