When you hang the tongue weight on back of TV, it adds weight to TV's rear axle and takes weight off from the TV's front axle. If too much weight is removed from the front axle, it can cause steering and control issues, including trailer sway.
The WD hitch uses leveraging to restore some (in some cases all) of the weight lost on the front axle. There is a small reduction in tongue weight, but, not enough to rely on it to not overload an undersized receiver.
Also note, tongue weight is not a constant number. It goes up and down during every trip. Water and groceries get loaded and used up. Holding tanks get filled. Depending on location (in relation to trailer axles) of holding tanks, they can raise or lower tongue weight considerably.
My trailer has a galley tank in front of the trailer axles, and a fresh water tank behind the axles. My black and grey tanks are directly over the axles. My tongue weight can fluctuate as much as 200 lbs during any trip.