Hi,
I have to respectfully disagree with many of the previous posters here.
First off, to me, "sway" is any movement of the trailer that is independent of the tow vehicle.
Any trailer that is on a ball hitch will naturally have some amount of sway given the right conditions (quick turns, wind, road conditions, loading, etc.). Friction hitches (i.e.,; the Reese and others) are for "Sway Reduction" (per manufactures description). They do not eliminate sway but just reduce the likelihood and severity of it.
A "Sway Eliminating" hitch (i.e.; Hensley and others) does just that, it stops all movement of the trailer that is independent of the tow vehicle ("sway"). All forces acting on the trailer, that don't come from the tow vehicle, are transferred to the tow vehicle so the two units can only move as one. In essence, the trailer is connected to the tow vehicle with a solid beam for all conditions except when the truck initiates a turn. For the non-believers, all I can say is to try it.
Safe towing,
Steve