THE PLT REPORTED THAT HE WAS BEGINNING HIS APCH TO A LANDING SITE AT AN ELEVATION OF 5200 FT WHEN HE FELT A BUMP IN THE TAIL ROTOR PEDALS. THE HELICOPTER BEGUN TO SPIN TO THE RIGHT & THE PLT FOUND HE HAS NO TAIL ROTOR CONTROL. AN AUTOROTATIVE LANDING WAS MADE INTO THE TREETOPS WHICH HE ESTIMATED TO BE 125 FT ABOVE THE MOUNTAIN TERRAIN. REPORTEDLY, THE MAIN ROTOR MAST SEPARATED AFTER CONTACTING THE TREES & THE HELICOPTER ROLLED INVERTED & STRUCK THE GROUND IN A NOSE LOW ATTITUDE. A SMALL POSTCRASH FIRE WAS EXTINGUISHED BY A PORTABLE FIRE EXTINGUISHER FROM THE HELICOPTER. BEFORE CRASHING, THE HELICOPTER WAS BEING USED ON AN EXTERNAL LOAD OPERATION. AN INVESTIGATION REVEALED THAT IT WAS CARRYING A 30 GAL BUCKET THAT HAD BEEN LOOSELY FILLED WITH FABIC STRAP. WITNESSES REPORTED THE ACFT WAS DESCEND AT APRX 40 KTS WHEN THEY OBSERVED 1 OR MORE OF THE STRAPS TRAILING FROM THE BUCKET. SUBSEQUENTLY, A STRAP BECOME ENTANGLED IN THE TAIL ROTOR & IT WAS OBSERVED TO HAVE STOPPED ROTATING.