DURING HELICOPTER INSTRUCTIONAL FLT A STRAIGHT-IN TOUCHDOWN AUTOROTATION WAS ATTEMPTED. THE PLT SAID WITNESSES OBSERVED THE TAIL BOOM COME UP INTO THE TIP PATH PLANE. THAT EVENT USUALLY OCCURS WHEN TOUCH DOWN IS MADE ON THE HEEL OF THE SKIDS & THE MAIN ROTOR IS STILL PITCHED AFT BECAUSE OF CYLIC INPUT FOR THE LANDING FLARE. AS THE HELICOPTER ROCKS FORWARD AFTER THE HARD LANDING ON THE SKID HEEL, THE TAIL BOOM WILL BE SEVERED BY THE DISPLACED MAIN ROTOR. FOLLOWING THE TAIL BOOM SEPARATION THE HELICOPTER ROTATED ABOUT 120 DEGREES TO THE LEFT.