cav551:
If you use the thing properly and keep the revs up then some idiot interpreting the satellite tracker data will decide that you have been wasting fuel and deserve a written warning.
I’m sure if you were to check the injector pulse width when using the exhaust brake then you would find that the injectors are actually closed and no fuel is being used. The engine is on overrun therefore the drive coming back from the axle/gearbox is enough to keep the engine turning and the injectors shut off to save fuel.
Same for a car when people think knocking it out of gear on a downhill will save them fuel, it’s actually using more fuel to keep the engine at idle than using the drive from the wheels to keep engine turning and the injectors can shut completely.