The flight instructor of the helicopter reported that, he directed the airplane rated pilot to conduct an autorotation with a 180° turn, followed with a power recovery. Abeam the departure end of runway, he moved the throttle lever from the FLY position to IDLE. While conducting the maneuver, the student pilot overshot the runway and aligned the helicopter with the parallel taxiway. Upon realizing the unacceptable position, they elected to abort the maneuver and perform a go-around. While the student pilot continued to fly the helicopter, the instructor inadvertently moved the throttle lever from IDLE, aft, towards the OFF position, then forward, to the FLY position. The engine experienced a total loss of power and the instructor adjusted the throttle to no avail. As the helicopter continued the descent, the student pilot requested that the instructor "get on the controls". The instructor left the throttle in the full power position, took the controls and conduct a power-off autorotation. The helicopter landed on the taxiway, skidded about 180 feet, departed the taxiway, onto the adjacent grass and came to rest in a drainage ditch.The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The Federal Aviation Administrator inspector who examined the helicopter at the accident site reported damage to the left anti-vibrator mount beam.