flyhardmo:
an engineer told me the reason for spinning the engine after shut down was to stop the main shaft from flexing once it had cooled down. I'd be happy for some experienced chap/chapette
Sounds like a phenomenon common on several engines. After an engine is shut down 10 to 60 minutes (depends on engine size & rotor mass), a convection stratification occurs within the case, meaning that the segment of rotor that stopped at 12:00 is a bit hotter than at 6:00.
This makes the rotor shaft bow slightly.
After another hour or two, everything cools down and temps equalize.
But if you attempt a start during this bowed-rotor period, it's gonna be out of balance for a few minutes. I've heard of pilots "tickling" the starter while shut down to suck a little cool air through, and to randomize the rotor position.