The private pilot and his passenger had just departed in a single-engine experimental helicopter when he heard a "loud pop" followed by a loss of engine power. The pilot entered an autorotation from an altitude of 75 feet and flared at approximately 40 feet above the ground to avoid hitting trees. The helicopter landed hard in a grass field and the skids were spread upon impact. The two occupants were able to deplane before the engine exhaust caught the surrounding grass on fire, and subsequently destroyed the helicopter. The pilot reported that the main drive belt, which he had been recently replaced, had failed. The helicopter wreckage was examined by an FAA inspector; however, the drive belt could not be located and it was assumed to have been destroyed in the post-impact fire.