THE INSTRUCTOR PILOT AND THE RATED HELICOPTER PILOT WERE PRACTICING TOUCHDOWN AUTOROTATIONS. AFTER A LANDING AND GROUND SLIDE, THEY BOTH NOTED VIBRATIONS, THEN SHUT DOWN THE ENGINE AND EXITED THE HELICOPTER. THE MAIN ROTOR BLADES HAD STRUCK THE TAILBOOM AND SEVERED THE TAIL ROTOR DRIVESHAFT. THE WIND AT 1450 CST WAS FROM 140 DEG AT 16 GUSTING 22 KTS.