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Castor Effect. The fixed angle of the hitch head causes this effect which helps keep the trailer inline with the truck. The hitch does not rely solely on the bars for sway control. The fixed angle hitch head, flexing bars, and latches all work together to control the sway. The heaviest I've had my trailer was a little over 8,500 lbs and the hitch worked just as well as it did when the trailer was 1,000 lbs lighter. They also make 2,000 lbs / 20,000 GTW bars.
The bars are built to flex so it's not unusual to see quite a bit of deflection in them. It is possible to permanently distort a bar by overloading it either through static or dynamic overloading. In that case the person would need to go to the next size bar up.
