"... the tail rotor, which will initially go berserk with the overspeed"
Why would the tail rotor overspeed if the engine to transmission shaft fails? The tail rotor is driven out of the transmission not the engine, right? As long as the MR is driving the transmission (in the absence of the engine input) won't the transmission just drive the TR at the prescribed ratio to MR %?
"Do not shut down the 206 engine for your auto, or you will lose all tail rotor control, get to the bottom with a big spin, sideways impact, stall flick spin crash burn die."
I don't understand that either. What's the difference between "shut down the 206 engine for your auto" and having an actual engine failure auto? The latter needn't spin crash burn die.
All the above is contingent on the main transmission unit driving the TR shaft as well as the MR shaft.