The flight instructor informed the private helicopter pilot under instruction (PUI) that the purpose of the practice autorotation was to demonstrate ways in which to reach a suitable landing zone in the event that an engine failure were to occur in an area with limited landing zones. The intention was to focus on the glide with a power recovery before the need to flare. The instructor entered the autorotation while the PUI observed, but was not on the controls. The instructor was demonstrating s-turns, beginning with a right 90-degree turn, followed by a left 180-degree turn, and ending with a right 90-degree turn to bring the helicopter back to the original heading. The rpm remained in the green arc or above during the entire maneuver. During the last right 90-degree turn, the helicopter passed through what the instructor estimated to be 200 feet above ground level (agl), where he decided to discontinue the autorotation due to insufficient airspeed (40 KIAS), and a higher than expected rate of descent, which lead to a lower than anticipated height above ground. During the time it took for the instructor to decide to abort the autorotation, the helicopter had descended to approximately 20-30 feet agl. The instructor initiated a flare, and he focused on keeping the skids level, but the right skid hit a dirt ledge adjacent to the proposed landing spot that was approximately 6 feet higher than the surface of the intended landing zone. As the right skid impacted the ground, the helicopter pitched forward, and the nose impacted the ground. The pilot reported no mechanical failures or malfunctions at the time of the accident.