The certificated flight instructor (CFI) and student pilot departed on an introductory instructional flight towards a training area where they performed basic flight maneuvers. They then flew to an airport where the CFI demonstrated a simulated forced landing which was to be followed by a go-around to a runway that was reportedly covered with about 12-14 inches of snow. During the demonstration, the CFI reduced engine power to idle from an altitude of 2,000 feet mean sea level and executed a descent to an altitude of about 5 - 6 feet above ground level at 60 knots indicated airspeed. The CFI did not increase engine throttle during this descent to clear the engine. The CFI applied full throttle during the go-around, but the engine hesitated and the airplane continued to descend. The main landing gear wheels followed by the airplane's nose contacted the snow. The left wing contacted the ground followed by the right wing. Both wings buckled, resulting in substantial damage to the airplane. The Federal Aviation Administration Flight Training Handbook states in the section under 'Descents (Maximum Distance Glides)' that during power-off descents, the engine should be cleared periodically to prevent excessive cooling and fouling.