THE PILOT PROVIDED HIS PASSENGER WITH A LOW ALTITUDE AERIAL SURVEY OF A RACE CAR ROUTE WHICH INVOLVED FLYING OVER MOUNTAINOUS TERRAIN BETWEEN 100 AND 150 ABOVE GROUND LEVEL. DURING THE FLIGHT THE HELICOPTER ENTERED AN AREA OF RAPIDLY RISING TERRAIN, AND THE LOW ROTOR RPM WARNING HORN SOUNDED. THE PILOT STATED THAT HE DID NOT IMMEDIATELY LOWER THE COLLECTIVE. HE STATED, HOWEVER, THAT HE ATTEMPTED TO REGAIN ROTOR RPM WHILE MANEUVERING THE HELICOPTER IN A LEFT TURN TO REVERSE COURSE. THE PILOT FURTHER INDICATED THAT THE HELICOPTER DID NOT HAVE SUFFICIENT ALTITUDE AND AIRSPEED SO HE ATTEMPTED TO LAND. THE PILOT ENTERED AN AUTOROTATIVE DESCENT. THE HELICOPTER TOUCHED DOWN HARD ON THE SLOPING TERRAIN AND ROLLED OVER. THE CRASH SITE'S ELEVATION WAS 6,300 FEET MSL. THE CALCULATED DENSITY ALTITUDE WAS AT LEAST 9,000 FEET.