THE CERTIFICATED COMMERCIAL PILOT/CERTIFIED FLIGHT INSTRUCTOR WAS CONDUCTING TRAINING WITH A SECOND PILOT IN HELICOPTER EMERGENCY PROCEDURES, THAT INCLUDED HYRAULICS OFF LANDINGS. ON THE THIRD LANDING WITHOUT HYRAULIC BOOST PRESSURE, AT AN AIRSPEED OF ABOUT 2 KNOTS AND A SKID HEIGHT OF ABOUT 1 FOOT, THE HELICOPTER BEGAN AN UNCOMMANDED TURN TO THE LEFT. THE PILOT ATTEMPTED TO COUNTER THE TURN BUT WAS UNABLE TO MOVE THE FLIGHT CONTROLS. THE HELICOPTER'S LEFT BANK ANGLE, NOSE DOWN ATTITUDE AND LEFT TURN CONTINUED UNTIL THE MAIN ROTOR BLADES STRUCK THE GROUND. THE LEFT FRONT PORTION OF THE FUSELAGE AND SKID TUBE THEN STRUCK THE GROUND. AN EXAMINATION OF THE HYDRAULIC SYSTEM, INCLUDING DISASSEMBLY OF THE MAIN ROTOR CONTROL SERVOS, FAILED TO LOCATE THE CAUSE OF THE MALFUNCTION. THIS IS THE SECOND RPTD HYD OFF FLT CONTROL SEIZURE THAT RESULTED IN AN ACCIDENT. THE FLT CREW OF BOTH HELICOPTERS WERE WELL QUALIFIED.