As part of my previous life I was fortunate enough to spend 6 years as an instructor at a fairly well known OEM training facility in Fort Worth.
Whilst doing in house recurrency in a 206 with a colleague who had been working there for 35 or so years I was asked if I had ever suffered an engine failure.
Upon my negative response the throttle was retarded and then placed in the off position. We did ten or so engine off landings, and, as I recall there is no noticeable difference performance wise between having the engine off or at idle. The biggest difference was having the engine out horn going off and AW Brown telling me I'd better not screw it up as a go around was now no longer an option.
I have had a partial sprag clutch in a 206. We were trying to adjust the autorotational rpm and no matter what we did the rpm wouldn't change, but would drop to the bottom of the green with the Nr and Np needles staying stuck together. It wasn't until after about the third engine shutdown that we noticed that there was no needle split. You couldn't turn the blades backwards, but by putting your hand down the exhaust, with very little pressure you could hold the Np wheels in place while turning the blades. What would have happened to the Nr with an engine failure is an unknown, but it probably wouldn't have been pretty.