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Technically it is called a combination valve.
It contains a warning switch, that turns on the brake light if there is a leak in one side. (does not block any fluid pressure to any brakes, just prevents fluid passing from front to rear, or rear to front in case of a leak.). And a hold off valve, that holds off the front brakes for the first few PSI of braking pressure, so the rear brakes can over come the springs, and start to apply.
On 1/2 ton and RC's, (Not 3/4 and 1 tons trucks), it also has a proportioning valve built in.

Here is a small video of the different functions.

Also a cutaway view of the two different valves.

Sometimes it does work better to carefully open a bleeder on the front, ot back while someone is applying moderate brakes, until he light goes out. But it is supposed to recenter itself once the two sides are back to having equal pressure.
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