Doug Palmer wrote:

The reasonings ranged from "you should not turn an engine backwards" to
"turning the engine backwards disarms the impulse coupling", to several
issues in between.


It doesn't "disarm" it, it just won't fire. This means than the engine
is less likely to start if the mags are hot (since the impulse coupling
aids in the starting). However, NEVER trust a propeller. Even turning
them backwards can generate a spark and even if the engine doesn't start
it can kick the prop over enough to do damage.

The argument against it is that people claim it hurts the vacuum pump.
While some dry pumps are designed to only turn one way, that's at
operating speed. The vanes are in there loose enough when it's
not spinning to not be a problem.

Except when absolutely necessary (like to get the tow bar connected)
you shouldn't be turning the prop at all. There's no good reason to
justify the dangers.