On July 23, 1998, at 1900 hours Pacific daylight time, an Enstrom F28A, N288Q, overshot the intended landing area and slid down a 30-degree hillside near Pleasanton, California. The aircraft sustained substantial damage. The certified flight instructor and dual student/owner, the sole occupants, were not injured. The local instructional flight last departed the Reid-Hillview Airport in San Jose, California, at 1830, and was en route to Livermore, California. Visual meteorological conditions prevailed and no flight plan was filed. The certified flight instructor (CFI) reported that he and the student were practicing off-airport landings. The student performed a pinnacle approach and landing. The CFI stated that the helicopter started to vibrate and move sideways so he rolled on power. By the time the helicopter reached the edge of the landing area, the rpm had decreased. The CFI reported that he was not able to reposition the helicopter to a higher area, so he pedal-turned to put the tail downhill, lowered the collective, and rolled off the throttle. The helicopter slid approximately 300 feet off the top of the hill and down a 30-degree slope. The main rotor blades contacted the ground but the aircraft remained upright. The CFI reported that there were no mechanical malfunctions with the aircraft prior to the accident.