This has just about all been said by instalments but:
RAF's magneto check procedure advice in my time instructing on pistons was as follows:
Immediately after start and at 850 RPM IDLE:
"Drop, no Stop" (no2 off).
Both ON (RPM recovers)
"Drop, no stop" (no1 off).
Both ON (RPM recovers)
Drop AND stop (both OFF).
Both ON (RPM recovers)
This proves that both mags are working. It also proves the single, combined rotary switch grounds out correctly in ALL positions. I recall at least one case where such a rotary mag switch became worn and left at least one mag live at "OFF".
At idle the amount of mixture going through the engine is very small, so that a "pop" in the exhaust was seen as a lesser evil than breaking someone's arm or worse because of a permanently live mag. Logically, the procedure is only about as stressful to the engine as what happens during normal engine start.
This check was repeated after flight, before finally shutting down on the mixture control, which makes it even safer to park, as all remaining mixture is burned rather than lurking in the manifold.
This was a separate check from the Run-up / power checks just prior to take off. During those checks, (@2200 RPM, max 150 drop per mag) if switching a mag off results in the engine cutting completely, leave the switch alone and let the engine stop. Do NOT switch the mag back on or you risk blowing the exhaust off or damaging the crankshaft through reverse twisting stresses because of the very sudden driving / driven / driving effect.
