THE COPLT WAS FLYING THE ACFT AS IT WAS BEING FERRIED AFTER MINOR MAINTENANCE. AS THEY WERE IN A DESCENT AND WERE APCHG THE DESTN ARPT, THE CAPT NOTED A LEFT ENG TORQUE FLUCTUATION OF 20% TO 30% & ELECTED TO SECURE THE ENG, ALTHOUGH NO YAWING WAS NOTICED. DRG SHUTDOWN, THE LEFT PROP DID NOT FEATHER & DRAG INCREASED UNTIL THE ACFT WOULD NOT SUSTAIN LEVEL FLT. AN ATTEMPTED RESTART OF THE LEFT ENG WAS UNSUCCESSFUL, SO THE CAPT TRIED TO FEATHER IT AGAIN. HOWEVER, THE LEFT PROP STILL DID NOT FEATHER. SUBSEQUENTLY, THE PLTS WERE FORCED TO LAND IN AN OPEN FIELD ON UNEVEN TERRAIN & THE ACFT WAS DAMAGED. AN EXTENSIVE INVESTIGATION WAS MADE, BUT NO CAUSE COULD BE FOUND FOR THE TORQUE FLUCTUATION, NOR COULD THE CONDITION BE DUPLICATED, HOWEVER, THE INVESTIGATION DID NOTE THAT THE AIR & GROUND PROCEDURES FOR ENG SHUTDOWN WERE SIMILAR. A VARIATION IN THE GROUND SHUTDOWN PROCEDURES ALLOWED FOR ENGAGEMENT OF THE START LATCHES.