A senior instructor gave me this explanation after hearing my (very dodgy) explanation for Overturn / Underturn turning onto north / south:
The compass needle is counterweighted on the north-seeking end (here in NZ at least). When you bank, the weight will tend to point down towards the ground due to gravity. (It will of course be almost held in alignment by the flux lines, but not quite!)
This bank - induced rotation causes the effect of Overturn North, Underturn South. Try it with a pen, lump of plasticine.
This also explains why acceleration errors do occur while taxying, but turning errors don't - no bank angle.
Works for me, but I confess I haven't tried it on a student yet...
O8