THE AIRPLANE NOSED OVER DURING A FORCED LDG IN A SOFT FIELD. THE CFI REPORTED THAT HE HAD INITIATED A SIMULATED ENGINE POWER LOSS AT ABOUT 2200 FT AGL BY EITHER PULLING THE MIXTURE CONTROL OR BY TURNING OFF THE FUEL SELECTOR; HE COULD NOT RECALL SPECIFICALLY WHICH METHOD HE USED. HIS INTENT WAS TO HAVE HIS STUDENT PRACTICE A SIMULATED FORCED LDG. THE ENGINE COMPLETELY LOST POWER AND THE STUDENT ESTABLISHED A GLIDE AND WENT THROUGH THE APPROPRIATE EMERGENCY PROCEDURES CHECKLIST. THE STUDENT REPORTED THAT HE SUBSEQUENTLY FOUND THE FUEL SELECTOR IN THE OFF POSITION AS HE WENT THROUGH THE CHECKLIST. THE SELECTOR WAS TURNED BACK ON, AND UNSUCCESSFUL ATTEMPTS WERE MADE TO RESTART THE ENGINE. EXAMINATION OF THE WRECKAGE REVEALED NO EVIDENCE OF ANY PREIMPACT MECHANICAL MALFUNCTION/ FAILURE. THE FAA FLIGHT TRAINING HANDBOOK (AC61-21A) INDICATES THAT A SIMULATED POWER LOSS SHOULD BE INITIATED BY RETARDING THE ENGINE THROTTLE.