The student was receiving instruction in autorotative techniques. The crew completed several autorotative descents terminating in power recoveries. The instructor then demonstrated an autorotation during initial climb at 50 knots, simulating an engine failure after take off. The helicopter was levelled and throttle and collective applied, but the instructor was unable to arrest the rate of descent and the helicopter landed heavily. Following a bounce, the left skid dug into the ground and the helicopter cartwheeled before coming to rest upright, facing in the reciprocal direction to the landing. The instructor commenced the practice autorotation after take off at too low a height/speed configuration. Despite the use of power, he was unable to reduce the excessive sink rate prior to ground impact.