On March 14, 1998, at 1640 hours Pacific standard time, a Robinson R22 Beta, N4079M, nosed over during a forced landing after takeoff at the San Luis Obispo, California, airport following an inadvertent engine shutdown. The aircraft sustained substantial damage, and the flight instructor and student pilot were not injured. Visual meteorological conditions prevailed and no flight plan was filed. The instructional flight was conducted under 14 CFR Part 91 and originated at 1634. According to the flight instructor, he was conducting an instructional flight with a student pilot who had logged approximately 6 flight hours. They were planning on doing traffic pattern operations to a grass area parallel to runway 29. The instructor stated that while they were on the downwind leg of the first pattern about 500 feet agl, the student was on the controls and was preparing for the approach with the prelanding checklist. The student reached down to apply carburetor heat, but instead released the mixture guard and pulled the mixture to idle. The engine stopped and the instructor reported that he then took over the controls and performed an autorotation to the grass field. The instructor stated that there was an excess of forward speed remaining at the completion of the maneuver, so the helicopter continued forward on the ground. The front portion of the skids became imbedded in the ground and the helicopter then nosed over. The blades hit the ground and the helicopter rolled over and came to rest on its right side.