Question: How does a under dash or trailer mounted brake controller work?
Answer: The brake controller is activated by the driver from his normal seating position when he brakes.
The brake controller is wired directly to the battery and directly to the brake lights. When the brake lights are activated, a contactor within the brake controller closes and allows current to flow directly to the brakes at a pre determined voltage.
The knob on the brake controller must be preset according to the trailers weight. Most trailers carry the same weight daily. Example, builders trailer, car carriers, pop tops, RV, etc
The more the trailer is loaded, just turn the knob to a higher voltage before you commence your trip. If weight is removed then turn the voltage down before you commence your trip.