During a check flight the pilot applied full power prior to conducting a timed climb. However, the engine rpm suddenly increased and it became apparent that the drive to the propeller had become disconnected. The aircraft was damaged in the subsequent forced landing, although the occupants were uninjured. It was found that the drive belt that connected the engine driveshaft to the propeller shaft, located immediately above the engine driveshaft, had migrated out of the groove in the lower pulley. This may have been the result of a degree of misalignment between the engine driveshaft and propeller shaft.