Gentleman, your discussion on freewheel drag has truly been entertaining... but it is a waste of time. Rate of descent is a function of density altitude, gross weight, and rotor RPM. anyone who has shut down a helicopter at the end of a flight knows that those blades continue to spin quite freely (as in "freewheeling"). The primary difference in the autorotation with and without the engine running, is the touchdown. With the engine at idle, the helicopter will fly the same down to the touchdown point, but... when you apply the collective and droop the rotor rpm for the landing, it will eventually meet back up with that idling engine. Most of the turbines I fly will idle around 90-92% power turbine speed. Once the rotor slows to that 90% speed, it couples with the idling engine and the droop will be drastically slower(0.5 - 2 seconds) in terms of how long we can cushion that landing. With the engine off, the rotor droop is faster and if you have never attempted this, it will feel as if the aircraft drops much faster during the touchdown since the rotor reaches the stall much faster. My speculation is that this professional entered autorotation and misjudged the approach, Stretched the glide, or got slow and pulled the cushion too early. Either way, the NTSB has video of the incident and I'm sure they will use sound analysis, video analysis, and all GPS data to rebuild exactly what occured.