All the 'armstrong starters' (sounds better than hand-swingers) among us will know that turning the engine over backwards, switches OFF and throttle OPEN is an essential remedy to an over-primed engine... so I doubt that the idea that doing so damages 'vanes in the magneto' holds much water!
Actually, the whole thing of turning the motor over before swinging the prop acts as a very useful diagnostic aid. As well as hydraulic locks in those awful old Gipsy engines (lower your guns chaps - I was only joking), you can keep an eye on compression and ought to be able to feel if summat's bust inside the crankcase.
One of life's simpler pleasures is sending the flashing-blue-lights-on-their-heads brigade into orbit by solo hand-starting the old L-4 Cub whilst standing behind the prop: it is safe, for reasons I've explained elsewhere, but they don't realise it is!