The flight instructor and the student pilot of the helicopter were practicing takeoffs and landings in the airport traffic pattern. During the downwind leg of the second pattern, while the flight instructor was performing pre-landing checks, the student pilot asked the flight instructor if he could activate the carburetor heat, to which the instructor replied in the affirmative. Immediately after, the engine lost power completely and the low rotor rpm warning system activated. The flight instructor and student pilot then realized that the student had inadvertently manipulated the mixture control. The flight instructor took then control of the helicopter, entered an autorotation, and aimed for an open field. During the landing, the main rotor blade struck the tailboom, severing it from the helicopter. While securing the helicopter, the instructor confirmed that the mixture control was pulled out, with the plastic guard around its base. The flight instructor reported that there were no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation.