The brake controller knows only 2 things: 1) whether the brake pedal is depressed (the controller gets it from the brake light sensor switch which turns on the brake lights in the back), and 2) an inertial sensor inside the controller detects when the car is either being decelerated (or pointed down hill, too).
Even the dumbest brake controllers know not to brake without both affirmative answers, but the smarter ones are also able to be more logical about things. The trick is to have the brakes working proportionally as you need them without overdoing it or underdoing it.
This thread from the forum library does a great job of explaining the details. Even though it was posted in 2005, it's still pretty accurate:
http://www.trailmanorowners.com/foru...ake+controller
The worst things about timed/progressive controllers: 1) They delay taking your braking actions seriously to avoid the situation where the trailer brakes are causing the deceleration sensor to think the braking is you, not it, and thereby braking even harder and going into it's own viscous cycle panic stop, and 2) In a long descent, if you were to apply the brakes lightly but continuously for a long time, the trailer brakes will keep braking harder until the trailer brakes are doing lots of the braking for both vehicles. While this does result in stopping, it can overheat the trailer brakes or put the trailer wheels into a skid (locked trailer wheels are something you really don't want, because locked wheels are just as happy going sideways as fore and aft). For Uhaul, I'm sure the timed controllers provide for easy adjustments, and the wild variations in trailer loads in the course of a weekend with a Uhaul trailer would make our carefully crafted Prodigies much harder for people to know how to use (unlike theirs, our trailers weigh pretty much the same all the time).
The best thing about the Prodigy and the P3 is that they also have you tell them how heavy the trailer is relative to the tow vehicle so that they can use that in a much smarter way to "read" the inertial sensor and make a much better guess about what's going on, and the trailer brakes are more effective.
Of course, trailer brakes are never as good as those wonderful traction controlled antilock brakes on our tow vehicles.