On February 29, 2008, at 0933 Pacific standard time, a Schweizer 269C, N958CP, sustained substantial damage during a forced landing on uneven ground while practicing pinnacle approaches near the North Las Vegas Airport (KVGT), near Las Vegas, Nevada. The certificated flight instructor (CFI) was not injured. The pilot under instruction (PUI) sustained a minor injury. The helicopter was operated by Airwork, LLC, of Las Vegas. Visual meteorological conditions prevailed, and no flight plan was filed for the local area instructional flight. In a written statement, the CFI reported that he and the PUI were practicing pinnacle approaches and had already completed a few high reconnaissance and low reconnaissance of a pinnacle. They proceeded to the next ridge line, and made an approach to it. During the last 50 feet of the approach, the rotor rotations per minute (rpm) started to decay as the PUI was raising the collective. The CFI advised the PUI to roll his throttle up. The CFI also reached for the throttle. The PUI was still pulling in pitch. Both rpm's decayed further. The CFI took full control at this point. He attempted to descend down the side of the pinnacle, but the helicopter was descending too fast. The CFI completed a steep slope landing, holding in full right cyclic. However, the helicopter began to slide backwards. The CFI repeatedly put in both right and left pedal, digging the skids into the ground to prevent further sliding. At this point, half of the collective was pulled in as well as full right cyclic (to the cyclic stop). The CFI determined that his only option was to put the helicopter into a hover from the slope, hover to the left and down the approximate 1,000 foot cliff, and then, with plenty of altitude available, attempt to gain airspeed and return rpm. The CFI then leveled the helicopter from the slope and started to go into a hover. Rotor rpm was barely into the green. As both skids came off the ground, the CFI allowed the helicopter to slowly drift to the left to get off the slope. The helicopter started to settle as he could not lower collective to roll the throttle on to gain power. The helicopter continued to drift to the left, and the tail stinger and tail rotor contacted something on the slope. The helicopter began an immediate and fast right yaw. The CFI rolled the throttle off to arrest the rotation. The helicopter stopped spinning. The helicopter landed softly, but the helicopter rolled over due to the slope. Both the CFI and the PUI exited through the passenger side door. The pilot reported no prior mechanical malfunctions. The pilot reported the weather at the crash sight as winds approximately 5 knots out of the west. An automated weather report indicated weather was clear with light and variable winds, not gusting.