WHILE ON THE DOWNWIND PORTION OF THE TRAFFIC PATTERN THE CFI INITIATED A SIMULATED ENGINE FAILURE. HE AND THE COMMERCIAL STUDENT PILOT STATED THAT JUST BEFORE TOUCHDOWN THE AIRPLANE'S RATE OF DESCENT INCREASED RAPIDLY. THE APPLICATION OF ENGINE POWER DID NOT ARREST THE RATE OF DESCENT BEFORE THE AIRPLANE LANDED HARD. THE AIRPLANE'S LEFT WING HIT THE GROUND FIRST, FOLLOWED BY THE LEFT MAIN LANDING GEAR. BOTH PILOTS STATED THAT AT THE TIME OF TOUCHDOWN THE AIRPLANE WAS IN AN APPROXIMATE 5 DEGREE LEFT BANK. THE LEFT MAIN LANDING GEAR SEPARATED AND THE LEFT WING HIT THE GROUND.