As we settled in I pointed out that I'd lost my checklist for the Jodel ages ago. "Good", he said. "Prefer you not to use them". Okay.
No, not okay. Your instructor is an idiot.
I always understood the master had to be off to ensure complete safety - in case of circuite failure in the mags somewhere.
No. The reason you have magnetos is because they produce their own electrical impulse, and are completely independent of the aircraft electrical system. Battery master switch on or off makes no difference. If you experience an electrical failure in flight with no aircraft electrical system available, it won't affect your magnetos. The only thing necessary for them to produce an electrical impulse is rotation.
The "circuit failure" that can occur in the magneto/ignition switch circuitry is a failure of the grounding "p lead" or "earthing lead" to the magnetos. If this occurs, the mag can be made to cause an electrical impulse, generating a spark at the spark plug, regardless of the ignition switch/mag switch position. This has nothing to do with the master switch, however.
We were quite nose-up when it happened: the nose dropped......and the engine died. Engine failure!
The prop was still turning, so I lowered the nose a little further to increase the revolutions, added a whisker of throttle - and she burst back into life!
It bumped back to life because you had propeller rotation, and magnetos producing electrical impulses for the spark plugs, as a result of that rotation.
It likely quit because of carburetion issues. More than one pilot has found himself without power when doing push-overs involving less than a positive G, causing the float in the carburetor to "float," sometimes sticking. The end result can be an unrecoverable power loss.
If you had carburetor ice, it was already forming; simply pitching up and then pitching down doesn't cause the problem. An ice buildup under extreme icing conditions, can happen quickly. Whereas you didn't report it during any other part of your flight, this was most likely not the case. Try less aggressive pitching next time, and allow a wing to fall off to unload the airplane rather than pushing the nose over. Your airplane isn't really meant to leave you floating in your seat.