It doesn't. The old style brake controllers were tied into a brake line, then based on how much brake line pressure there was, it varied how much power it fed to the electric brakes on the trailer.
The only brake controller options now is a pendulum system that varies the braking power based on inertia and a timed delay version which waits, then applies a pre-set amount of braking power.
I personally feel that the timed delay controllers should be made illegal. They work well for towing until you get into an emergency situation where you need to make a panic stop. It will not assist your braking for the set time delay, and then does not apply full braking power like the old hydraulic and newer pendulum critters do.