Full Report:

On October 24, 2015, about 1640 Pacific daylight time, an American Eurocopter AS350B3, N911WL, landed hard during a practice autorotation near Folsom, California. The helicopter was registered to Placer County, and operated by the Placer County Sheriff's Department, as a public aircraft flight. The flight instructor (CFI), pilot undergoing instruction, and tactical flight officer (TFO) were not injured; the helicopter sustained substantial damage. The training flight departed Mc Clellan Airfield, Sacramento, California, about 1600. Visual meteorological conditions prevailed, and a company flight plan had been filed.
The purpose of the flight was to perform patrol missions, in addition to providing training for the pilot undergoing instruction. He had recently been hired by the sheriff's department, and this was his first patrol training flight. He was seated in the front left seat, with the CFI in the front right, and the TFO in the rear jump seat.
The crew departed from their base at Auburn Municipal Airport about 1230, and reported that the flight, which included a series of autorotations to power recovery, was uneventful. They then landed at Mc Clellan Airfield, and serviced the helicopter with about 120 gallons of fuel. After departure they flew to a peninsula on the northern shore of Folsom Lake with the intention of performing more autorotation training. They surveyed the area and began the maneuver at 2,500 ft mean sea level (about 2,100 ft above the lake). The CFI stated that he was the pilot-in-command, and that the pilot undergoing instruction was "shadowing" the controls as the CFI demonstrated how to maintain rotor rpm during the descent.
The descent was uneventful, and the CFI stated that as they approached 100 ft above ground level (agl), he verbalized his intentions to perform a power recovery, and began by turning the engine control from idle to flight, while simultaneously initiating the flare. As the flare progressed, the pilot did not hear the engine fully regain its speed, and he checked the instruments observing that the engine and rotor rpm needles were still "split." He prepared for a full touchdown landing, but by now the helicopter had moved forward to an area of downsloping terrain. The CFI pulled the collective to cushion the landing, and the helicopter landed hard and flat on the skids. The engine had now recovered to full operating speed, and the CFI lifted the helicopter back off the ground and landed it about 100 ft ahead. After landing, he discovered that the tailboom had bent downwards at its intersection with the aft bulkhead.
The helicopter was recovered from the accident site the following day under the supervision of the sheriff's department, and the accident was reported to the NTSB 3 days later.