On May 12, 2006, about 1525 central daylight time, a Schweizer 269C helicopter, N7506L, piloted by a certified flight instructor (CFI), and a commercial pilot, was substantially damaged when it landed hard and rolled on its side during a practice autorotation near Lees Summit, Missouri. The training flight was operating under 14 CFR Part 91 without a flight plan. Visual meteorological conditions prevailed. There were no injuries. The local flight originated from the Lees Summit Municipal Airport (LXT), about 1500. The CFI reported that his student was training for an add on helicopter CFI rating. He reported that they had performed three touch down autorotations at LXT earlier in the flight. The CFI stated that he and his student were performing a practice autorotation when he realized that they would not be able to clear the trees at the edge of the landing area. He stated that he took control of the helicopter and attempted to add power but could not due to resistance in the throttle system due to his student not relinquishing control. He stated that he may have pulled some collective in order to just clear the trees, and then flared the helicopter. He said that the helicopter impacted the ground in a level attitude and eventually came to rest on its left side. He reported that there were no mechanical failure of the helicopter.