The flight instructor stated that he and his student were on downwind at an altitude of 500 feet, preparing to land the helicopter. He further stated that as they performed prelanding checks, the student pilot within the space of a second removed the guard and pulled the mixture, causing the helicopter's engine to cease operating. The instructor said he immediately entered autorotation, and further stated that the wind was coming from the left, i.e. from the south, and there was no place to turn left due to trees and a road, so he turned right. He said he then saw some trees so he continued to turn right to an open grove, and then started "full down." The student was depressing the right pedal, and the instructor said he continued to maintain rpm as high as possible for the full down, pushed forward, and affected the landing. He said the helicopter landed and continued with its nose to the left due to the right pedal being depressed by the student, and it rolled slightly on its right skid, then collapsed, and came to rest on its belly.