That's a governor smoke test, which isn't really needed on an engine with electronic fueling controls.
A modern diesel makes most smoke under acceleration, which is why the MOT regs now call it an acceleration smoke test. Most MOT test stations won't risk an engine failure for carrying out a pointless test governor smoke test, when a simple acceleration test is all that's needed.
That's not what it says on the gov website

Hold the engine at full fuel position until a release prompt is given and immediately release the accelerator pedal.
 
That's not what it says on the gov website

Hold the engine at full fuel position until a release prompt is given and immediately release the accelerator pedal.
I've asked my MOT tester in the past as to why they don't hold it at the governor any more. He told me that a governor test was the cause of too many engine failures, and isn't needed on the new smoke test equipment, so they don't do it.
Many modern engines don't rev to maximum RPM off load anyway, the wife's Audi only revs to 2250 RPM off load, which I suspect is a safety protocol built in to protect the engine.
 

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