It seems we had a long-winded discussion about this a while back. I will try to explain briefly.
The "water brake" is sometimes also called the "drifting throttle", and although there are differences, to use is the same.
With the water brake, a small amount of water is tapped off the boiler and fed into the valve chests of the locomotive. As this water is about 350 degrees (when boiled at 200 psi) it flashes into wet steam at a lower pressure as it goes down the pipe to the valve chests. The locomotive is placed in reverse motion. The steam acts as a compression brake against the pistons. The idea is the wet steam and low pressure doesn't destroy the lubrication and damage the locomotive when it is running in reverse.
The more modern superheated locomotives used drifting throttles. This is saturated steam pulled off the turret ahead of the cab through a valve and piped into the valve chest. As before the locomotive is placed in back motion and the steam is used as a compression brake. Care must be taken as to not overheat the cylinders as the mechanical lubricator that pumps oil into the valves and cylinders cannot pump against the high pressure produced in the cylinders.
Which brings up several points in LeMessena's article which I had trouble with. He stated that the the water brake was used to bring trains downhill instead of airbrakes, which couldn't have been done. The valve and piston rings would have been eaten alive. If 3 3600's hefted a train up Tennessee Pass how is one 3600 going to hold it back on the downgrade with saturated steam? Yes, the water brake/drifting throttle is a big safety net when the air starts getting thin and the train is handling poorly and would certainly be useful in holding the train back while you tried to pump up the auxiliary reservoirs (done this many times), but to use it as the primary braking force won't work.
Another problem is very heavy buff forces on the head end when all the braking power is on the front. This would tend to shove cars out of the head end of a long heavy train the same way using too much dynamic brake will do today.
