HFD. I am not sure, but I recall the de-tuning of the counterweights (must admit to ignorance on how they get buggered up) can also be caused by rapid throttle closure. In another words a snap closing of the throttle lever.
Page 50 of the Lycoming Flyer Reprints says this:
"Many flight instructors down through the years used the technique of abruptly cutting an engine with a multiengine candidate to test his emotional reaction and judgement with this extreme technique. (my comment is that testing emotional reaction is the job of a trained psychologist - not an enthusiastic amateur) Big radial piston engines with short stubby crankshafts could tolerate the abrupt technique. However, flat opposed piston engines with their long crankshafts and attached counterweights could not as readily take the abuse of suddenly snapping a throttle shut, particularly at take off or climb power. Use of the latter technique would tend to detune crankshaft counterweights and could possibly result in a nasty engine failure. Since it was common technique by flight instructors to terminate power abruptly to simulate an engine power loss, we had to protect the engine. As a result we published in our Engine Operator's Manual and in Service Bulletin No. 245, the recommendation that if power was abruptly terminated, it must be accomplished with the mixture control. Of course, this was intended for higher altitudes where a complete engine shut down could be conducted safely.
The student was to identify the dead engine by retarding the throttle to about 12 inches MP to simulate zero thrust, or similar to having the prop feathered. At that point the instructor could immediately return the mixture to an engine operating position and power would be available if needed. However, any practice of simulated engine out condition at low altitudes should be best accomplished by a slow retardation of the throttle in accordance with the NTSB recommendation. This careful technique will protect the engine and at the same time provide for instant power if it is needed."
End of quote.
For instructors the problem is getting hold of a copy of the Engine Operator's Manual and Service Bulletin No. 245. This of course is the CFI's responsibility to obtain these documents and ensure his staff read them. It probably never happens and techniques (faulty, or hopefully correct) then get passed down the line from instructor to student and so on indefinately and that is how myths are propagated.