Heavy trailers hitched behind the rear axle push the back of a tow vehicle down and lift the front. A weight distributing hitch exists to counter that geometry, and it is not a towing capacity upgrade.
Tongue weight acts through a lever
The hitch sits behind the rear axle, so tongue weight applies a downward force at the end of a lever with the rear axle as its pivot.
That levers the front of the vehicle upward, taking load off the front tires. Both steering authority and front braking depend on that load being present.
The rear suspension compresses at the same time, which changes headlight aim upward and reduces the suspension travel available for bumps.
Spring bars apply a counter torque
A weight distributing hitch adds two spring bars running from the hitch head back to brackets on the trailer frame, tensioned so they resist the downward tongue force.
That tension applies a torque opposing the lever effect, pushing the hitch head up and effectively transferring load forward through the vehicle's frame to the front axle.
Some load also transfers rearward to the trailer's own axles, which is why the system is described as distributing rather than merely relocating weight.
Setup is measured, not estimated
Correct adjustment is verified by measuring front and rear fender heights unhitched, then hitched, and adjusting bar tension until the front returns close to its original height.
Over-tensioning is a real fault. Removing too much weight from the rear axle reduces rear traction and can make the vehicle feel light and unsettled.
Bar rating must match the actual tongue weight. Bars sized for a much heavier load cannot be tensioned finely enough for a light trailer.
Sway control is a separate function
Many weight distributing hitches include friction or cam-based sway control, but distributing weight and resisting sway are different jobs achieved by different parts.
Sway originates in trailer loading and speed, and the primary fix is placing enough of the trailer's weight ahead of its axles rather than adding hardware.
Integrated systems combine both because a badly balanced trailer that also unloads the front axle produces the worst handling case.
What the hitch does not change
Vehicle towing and tongue weight ratings are set by the manufacturer, and a weight distributing hitch does not raise them unless the manufacturer publishes a higher rating with one fitted.
Some manufacturers do publish exactly that, requiring weight distribution above a specified trailer weight. That is a condition of the rating, not a bonus.
Brakes, cooling and frame strength remain the limits. The hitch corrects load distribution, which is a handling and control problem rather than a capacity one.