The certified flight instructor (CFI) was giving flight instruction in the piston engine powered helicopter to an experienced airplane pilot, who had a total of about 2 hours of helicopter flight experience. The pilot receiving instruction was hovering the helicopter while oriented into the wind and he allowed the rotor RPM to decrease. While the helicopter was turning to the left, the CFI took over control, but was unable to regain RPM. The helicopter continued to turn to the left and the CFI conducted a downwind emergency running landing to soft muddy turf. After a ground run of about 15 to 20 feet the helicopter began to turn to the right and skids of the helicopter dug into the soft turf. The helicopter rolled 90 degrees onto its left side and both main rotor blades impacted terrain. The helicopter sustained substantial damage to the main rotor system, transmission, tail rotor, tail boom and fuselage. The CFI reported that he had been too slow in taking remedial action. A postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The characteristics displayed by the helicopter were indicative of it encountering a dynamic rollover.