DURING AN INSTRUMENT TRAINING FLIGHT, A COMMERCIAL PILOT RATED STUDENT & AN INSTRUMENT INSTRUCTOR PILOT (CFII) FLEW TO DULUTH TO PRACTICE APPROACHES, THEN RETURNED TO THEIR HOME BASE AT SUPERIOR. FBO PERSONNEL AT SUPERIOR HEARD THE CREW MAKING AIRPORT TRAFFIC ADVISORIES, WHILE MAKING APPROACHES AT THE RICHARD I BONG AIRPORT. SUBSEQUENTLY, THE AIRCRAFT CRASHED ABOUT 1.6 MILES SOUTHEAST OF THE AIRPORT. GROUND IMPACT OCCURRED IN A STEEP DESCENT. A MOTOR VEHICLE DRIVER SAW THE AIRPLANE SPINNING AS IT DISAPPEARED FROM HIS VIEW BEHIND A TREE LINE. EXAMINATION OF THE WRECKAGE DISCLOSED NO MECHANICAL DEFECTS. THE AIRPLANE'S LANDING GEAR AND FLAPS WERE EXTENDED AS IF FOR AN APPROACH. THE CFII WAS GIVING MULTI-ENGINE INSTRUMENT INSTRUCTION TO THE COMMERCIAL PILOT IN PREPARATION FOR TESTING TO REMOVE A VFR-ONLY RESTRICTION FROM HIS MULTI-ENGINE RATING. A MULTI-ENGINE INSTRUMENT FLIGHT CHECK WITH A DESIGNATED PILOT EXAMINER WAS TENTATIVELY SCHEDULED FOR LATE IN THE AFTERNOON, AFTER THE INSTRUCTIONAL PERIOD.