That wouldn't be possible with the engine still running. Start locks can only engage with negative blade angle AND engine speed low enough for the springs to overcome the centrifugal force, i.e. during shutdown.
Simulating an engine failure on a Garrett is a critical exercise and I doubt you'd find any check pilot these days that would retard the PL to less than zero thrust (roughly 20% torque), or too quickly. Must be something else at play here.
On the ATSB website you will find a report of how an instructor managed to do what you say is not possible on a Brindabella
Airlines Metro over Lake George. Fortunately the "student" had the presence of mind to pull the fuel shutoff handle to shut it down.