Exhaust brake will do the same thing as manually putting your truck in a lower gear when coming down hill. It will hold your truck in whatever gear you are in and not allow an upshift to a higher gear, thus allowing the truck to move even faster. With exahaust brake, the engine will rev, it can get pretty high and sometimes can become a bit daunting, but that's what's it's suppose to do. When the engine reves that high, then you press the brake pedal and slow the entire rig down, sometimes, even enough to drop into the next lower gear. But normally, it will just rev the engine maybe 1000 - 2000 rpms higher than normal running.
The thing it does is keeps your transmission locked in the current gear so it won't upshift when coming down hill. It does not hurt anything to engage it the moment you start your engine (towing or not). I've used mine in the mountains of NC, Tennessee, and KY, even when not towing. But towing is where it really shines.
Now, the opposite is Tow-Haul mode. Tow Haul cause the transmission to shift up to the next highest gear at a higher engine RPM.
Cruise control holds the speed steady.
Now, turn on all 3, and you virtually never have to touch the brake or the accelerator once you reached your desired speed.
Tow-Haul will keep the RPMs up when climbing hills so your truck maintains max power in a lower gear. Exhaust Brake keeps the truck in it's current gear when going down hill so the lower gear is actually keeping you from accelerating. When Cruise Control is flipped on, everything just happens! It's so sweet!