The pilots' incorrect activation of the Manual Override Lever, during ground operation, in an attempt to correct a sub-idle speed condition of the engine, resulting in an over-temperature of the CT blades, their subsequent distress and failure of the engine. Contributing to the incident was: The incorrect guidance of the Piper Meridian Pilots' Operating Handbook which, contrary to the engine manufacturer's recommendation, allowed the operation of the Manual Over ride Lever during ground operation.