The OP didn't ask whether it should be done - the question is why or how it works as a brake.
Answer - it is like putting a car or truck into low gear to "engine-brake" going down a hill. The aircraft is absorbing energy from the slipstream/airspeed, in order to drive or spin an idling engine as an air pump instead of a power source. Instead of the engine driving the prop and adding energy to the propwash.
AND also producing drag directly by making the prop blades a "flat disc" - the opposite of feathering the blades to reduce prop-engine drag.
In either case it makes it harder for the aircraft to move through the air, thus reducing speed, or holding a speed while being able to increase descent rate.
It can be hard on the engine, because it is in effect trying to make the drivetrain run backwards (which does not happen literally - it is just that the forces are trying to do that). I.E. producing "negative torque" in the various moving parts. It can be especially hard on a reduction gearbox, as in turboprops.
Generally speaking the aircraft/engine manuals will provide guidance on just how much this effect can be used - if at all. MP and rpm limits for pistons; torque for turboprops, maybe engine temps for both.
As mentioned, a torque sensor may be installed, to overrule you as the pilot, unflattening the blades automatically to avoid negative torque. Or you may be left to watch the torque gauge yourself to keep it positive.
On the whole, I agree that unless one has screwed up the descent planning, it is better to leave the props in cruise pitch (higher torque), which leaves more elbow-room to reduce power (fuel flow) without nudging too close to zero or negative torque. Eventually slowing to flap or gear speed - where one can add drag using those, instead of straining the engines shafts and gears. Once that drag is in, THEN one can flatten the blades to full forward on final, for speed reduction and preparation for a possible go-around, while still using substantially more than idle power (i.e. substantially more than zero torque).