Centaurus,
Training with such engines was always a disciplined process. The check pilot would fail the engine using the mixture. The pilot would confirm by retarding the throttle knob one knob width and doing the same with the pitch lever. The check pilot would then slowly close the throttle, enrichen the mixture and set zero thrust.
The only failure I suffered in a GTSIO was shortly after take-off with the gear retracting. The cause was a snapped gudgeon pin. Myself and seven pax had a near death experience avoiding a number of obstructions around the circuit. That aircraft had been used a few days previously by a DCA examiner who insisted on your technique during an instrument renewal!
Textron Lycoming Mandatory Service Bulletin No 245D
“Rapid opening or closing of the throttle can cause counterweight detuning…To avoid detuning during simulated engine failure, use the mixture control to shut off the engine and leave the throttle in normal open position until the engine has slowed down because of lack of fuel. Then, close the throttle to an idle condition. The throttle being open allows the cylinder to fill with air, maintaining the normal compression forces which are sufficient to cushion the deceleration of the engine. Another result of rapid throttle movement is severe strain on the supercharger gears and associated gears because of the inertia force of the high speed impeller.”
Source
http://www.prime-mover.org/Aviation/.../sb245d-1.html