At the conclusion of an uneventful instrument flight rules flight, the flight crew of the business jet conducted an instrument approach to the runway. As the airplane was about to touch down, the pilot flying reduced engine power, the airplane descended and landed "hard" on a grassy area short of the runway threshold. The fuselage was substantially damaged during the landing. The pilot reported that there were no mechanical malfunctions or failures of the airplane what would have precluded normal operation.