Perhaps we should really address the initial problem -- the interpretation that lightening the tongue load by use of the WDH causes sway. Lets ignore the WDH for the moment. The advice on tongue weight is somewhere around 15% of the total weight of the trailer. What does this number represent? It is the weight that results from how the trailer is loaded, i.e. what percentage of the weight is in front of and behind the axle(s). If there is an equal amount of weight in front and back of the axle, there will be no tongue weight. If you use the often quoted 60% front/40% rear loading, in most cases you probably end up with about 15% of the total trailer weight on the hitch. So people use hitch weight to indirectly (and probably unknowingly) achieve the 60%/40% weight distribution. It's not the light tongue weight, per se, that causes sway. It's the improperly loaded trailer with too much weight in the back that is the problem. So regardless of what the WDH does or doesn't do with the tongue weight (tranfer, distribute, transmit, or somehow eliminate it all together), it doesn't effect the load balance of the trailer and the subsequent effect on sway.
My guess is that except for a very small amount of additional friction from the bars, WDH's in general have little effect on causing or decreasing sway. That's why we have Dual-Cams, friction devices, etc. as add-ons (except in the case of those units, such as Equal-i-zer, where the anti-sway is built in to the design).
