My proposed summary:
A weight distribution (WD)system distributes some of the weight or load from the rear axle of a tow vehicle to the front axle and to the trailer's axle(s). It does this by the force of the tensioners or spring bars applying leverage. A WD system does not lighten the tongue and does not change the percentage of a given trailer's tongue weight (measured prior to connection with a tow vehicle). The recommended tongue weight percentage range is 12% to 15%.
Think of a WD system as re-leveling the tow vehicle and trailer. When the trailer tongue is connected to the ball of a receiver, the rear axle bears much of the tongue weight. This raises the front of the tow vehicle and lowers the front of the trailer. The WD system works to distribute that rear axle load in order to re-level the tow vehicle and trailer. This will reestablish the tow vehicles original axle load ratio as designed by the manufacturer as to provide normal steering and braking control. A WD system does reduce the load on the receiver and by doing so increases the capacity of the receiver.
A WD system does not necessarily provide sway control unless the manufacturer incorporates this into the design.
Use the following link for a detailed explanation of how a WD system works, by Ron Gratz and an interesting discussion on the topic.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/14265335.cfm
