The pilot reported that he elected to practice a couple power recovery autorotations before landing at his destination. The first autorotation was uneventful. During the second autorotation he decreased the throttle to what he thought was the idle position. Shortly thereafter, he received an “engine out” audio warning. He glanced at the throttle position on the collective and observed that he accidentally had the throttle in the off position instead of the idle position. The pilot attempted to restart the engine, but with the ground quickly approaching, he elected to perform an engine out autorotation to the ground. He flared about 35 feet above the ground. About 7 feet above the ground, the main rotor RPM became too slow, and the helicopter dropped to the ground. The rear skids impacted hard; the helicopter rocked forward then came to rest on its left side. The helicopter sustained substantial damage to the main rotor system, fuselage, tail boom, and the tail rotor system. The pilot reported there were no mechanical failures or malfunctions with the airframe or engine that would have precluded normal operations.