The flight instructor (CFI) was demonstrating a 180-degree autorotation to a student pilot while performing an instructional flight. The CFI reported entering the autorotation when, at about 100 feet agl, the indicated airspeed suddenly dropped to about 40 knots. He pushed the cyclic forward in an attempt to regain airspeed and flared at about 50 feet agl in an attempt to slow down the helicopter's rate of descent. The helicopter landed on the edge of the runway on the back of the skids, bounced back into the air, and began an accelerated yaw to the right. According to the CFI, he lowered the collective and reduced throttle while the helicopter spun four or five times. The helicopter subsequently impacted the ground, and came to rest on its left side. A postaccident examination by a Federal Aviation Administration inspector revealed substantial damage to the cabin and the tail boom. The CFI reported no preaccident mechanical malfunctions or failures that would have precluded normal operation.