Indeed, the Olds Toronado demo illustrated that a weight DISTRIBUTING hitch takes weight from the truck rear axle
and distributes it between the front axle and trailer axle, which is why they improve handling (esp in FWD vans) by putting the front wheels back on the ground properly.
How much weight? Depends on how you set it with the chain links or whatever.
What proportion between truck front and trailer? Prolly depends on distance from front tire and trailer tire to hitch head or bar bearing point, using a calc base on foot-pounds, were the distances are in feet and the pounds are the lessened rear axle weight.
Want to know more? Call an engineer at Reese or DrawTite or Equalizer (Info might be available on web site).
If you already have a WDH, next time you are at scales, weigh front and trailer with and without hitch set, and weigh rear axle with and without WDH set. All the numbers should add up all the time, just distributed differently.
Pete and Rats