In a turbine helicopter, there is generally an output shaft from the engine which is connected through a freewheeling unit to the main rotor transmission. The freewheeling unit is there to allow the main rotor system to continue to spin if the engine stops. However, a failure of the freewheeling unit, or of the driveshaft, or any other components between the engine output and the main rotor transmission input could prevent the engine from being able to turn the main rotor.
Generally the pilot would identify a short shaft failure by his engine RPM tachometer showing a rise in RPM (because the engine is suddenly unloaded) while simultaneously seeing a decrease in his main rotor RPM (because it is no longer being powered by the engine).
The recovery is to treat the failure as an engine failure. The pilot would enter autorotation in order to keep the main rotor turning, and would also move the engine control to idle to prevent a further overspeed of the engine.
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