The certified flight instructor (CFI) was preparing the flying pilot for his commercial helicopter check flight. They were practicing pinnacle and confined area landings to a small elevated Forrest Service landing platform located along a 4,750-foot mean sea level (msl) ridge line. The helicopter performance tables indicated, based on helicopter weight and environmental conditions, a maximum out of ground effect hover altitude of 6,000 feet msl. The pilot made a very shallow approach to the landing platform, and established a hover. At this time the engine manifold pressure increased, and the low rotor rpm horn sounded. The helicopter settled onto the landing platform and the flying pilot said to the CFI that the landing skid on his side was not on the platform. He maneuvered the helicopter forward off the platform, and downslope. The low rotor rpm horn stopped sounding, and the helicopter increased forward speed but did not increase in altitude. The right skid caught some snow and the helicopter flipped over. The helicopter came to rest facing uphill on it's left side with damage to the tail boom and main rotor blades. The CFI stated that the helicopter had no mechanical malfunctions or failures during the flight.