My understanding of how a weight distribution hitch works is: magic (leverage) is used to reduce the amount of weight pushing down on the hitch by transferring that weight back onto the axels of the towed trailer.  When the system is set up correctly on our rig, the rear of the truck only sags approximately 1/2-inch from its normal ride height.  Like I said, magic.  Though not listed as a selling feature, the W.D.H. has provided a small amount of sway-control.