WHILE RETURNING TO THE AIRPORT FROM AN INSTRUCTIONAL FLIGHT, THE ENGINE 'HAD A SUDDEN POWER DROP OF 1300 TO 1400 RPM,' ACCORDING TO THE CFI. THE CFI 'WENT THROUGH THE EMERGENCY CHECKLIST' AND 'COULD NOT HOLD ALTITUDE.' THE ENGINE LOST ALL POWER WHEN THE LEFT MAGNETO WAS SELECTED. THE CFI PERFORMED A FORCED LANDING ON SHORT, SOFT, HILLY TERRAIN, AND THE AIRPLANE NOSED OVER. THE FLAPS WERE FOUND IN THE RETRACTED POSITION, ALTHOUGH THE FLAP SELECTOR SWITCH WAS FOUND IN THE FULLY EXTENDED POSITION. AN EXAMINATION OF THE AIRPLANE DID NOT REVEAL ANY MECHANICAL MALFUNCTIONS. BOTH MAGNETOS WERE FUNCTIONALLY TESTED AND MET MANUFACTURER'S TOLERANCES. THE TEMPERATURE AND DEW POINT WERE 38 AND 20 DEG, RESPECTIVELY. ACCORDING TO CARBURETOR ICING PROBABILITY CHARTS, CONDITIONS WERE CONDUCIVE TO VISIBLE ICING AT GLIDE AND CRUISE POWER; THE CARBURETOR HEAT WAS FOUND IN A MIDWAY POSITION.