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Why do the jet turbines spin while the engines are turned off? Is this as a result of the wind? Or are the turbines deliberately spun using the APU? If so what is the reason behind this?
After shutting down they still have momentum. Or like you said, because of Windmilling - freely turning when the wind impacts the compressor blades.
The APU can, but more often than not doesn't, rotate the engines (overly simplified, not to go into unrelated details).
Required, but never shown