Robinson Safety Notice SN-27 of June 1994 is titled SURPRISE THROTTLE CHOPS CAN BE DEADLY and advises flying instructors how they should give a student a simulated power failure. It says:
"Never truly surprise the student. Tell him you are going to give him a simulated power failure a few minutes before, and when you roll off the throttle, loudly announce 'power failure'."
The instructor is also advised to plan to initiate recovery within one second, regardless of the student's reaction.
The safety notice came about because surprise throttle chops were killing people.
As to the rollover statistics off-airfield (Soggyboxers) that's deduction from long discussion with high-time pilots. The last for-real engine-off I heard about was the Puma in Oxfordshire last month. Full crew, supremely competent, and it rolled over. No reflection on the crew - that's just the shape of the earth.
As to doing as little damage as possible, yes, but don't make it an article of faith. If the helicopter's let you down, stuff the damned thing. Frank will make you another one.
Unfortunately, the ones we smash up were usually in perfectly good nick, and we break them practising the wrong end of the auto.