On May 8, 2000, at 0925 hours Pacific daylight time, a McDonnell Douglas 500N helicopter, N955SD, sustained substantial damage while performing off-airport landing training in Los Angeles, California. The helicopter was being operated by the Los Angeles County Sheriff's Department as a public-use aircraft under the provisions of 14 CFR Part 91. The commercial rated flight instructor and commercial rated pilot trainee were not injured. The local area instructional flight originated at the Long Beach, California, airport, about 0830. Visual meteorological conditions prevailed and no flight plan was filed. The flight instructor reported that the purpose of the flight was to practice emergency procedures as part of routine intradepartmental training. They were conducting the training in an open field on property owned by the Sheriff's Department. The flight instructor stated that they often utilized the field for flight training purposes, and reported that he had done some training there about 1 week prior to the accident. He reported that they performed a high reconnaissance of the field prior to landing; at that time he noted nothing unsuitable about the terrain. The flight instructor stated about 10 to 12 feet above ground level (agl), he reduced the throttle to ground idle to simulate a total power loss. The pilot trainee was on the controls; he landed the helicopter with no problems. The flight instructor stated that the helicopter slid forward only a few feet, then stopped abruptly. The skids had become entangled in 18-inch-tall grass and imbedded vines, causing unexpected friction. He stated that the natural tendency to input aft cyclic to stop the aircraft from going over caused the main rotor blades to contact the tail boom. The flight instructor reported that neither he nor the pilot trainee had seen the high vegetation before landing, nor had they ever noted it there in the field before.