The private pilot was flying his homebuilt airplane on a cross-country flight at 5,500 feet mean sea level when he encountered some turbulence. The engine began to overheat and lose power and the pilot diverted to a nearby airport. The pilot could not make the runway and landed on a grassy, unimproved area on the airport. The airplane struck a pole, crossed a road, struck a second pole, and came to rest inverted. Both wings and a portion of the nose were separated from the airplane and the pilot was seriously injured. The pilot stated that he thought he had inadvertently actuated the landing brake in the turbulence, which blocked airflow to the engine and caused the engine to overheat and lose power. According to the builders manual, deployment of the landing brake can disrupt airflow to the engine and cause overheating. The airplane had been modified from the original design, replacing the mechanical landing brake system with an electrical one.