This is probably better in the Tech log forum. For how a Mag works you are probably best going into the books, but basically it generates an electrical charge by rotating a magnet inside a coil to give a spark which is supplied to the spark plugs (obviously more complex than this). Magnetos are by their nature always live and when you switch to "Left" you are actually earthing (in effect turning off) the right magneto, this means off course that if the wiring to the switch is defective they will normally fail in the "live" position - the danger here of course being that this leaves a live propellor and is why props should always be treated as live. Normally of course you have two sets of plugs which are supplied by the seperate magnetos and if one magneto fails you will still have a working engine, though not quite as efficient - this is what is happening during the mag check, only one set of plugs is being supplied. Should both fail of course no spark plugs are being supplied and the engine will stop. One other major point to note here is that magnetos are independant of the aircraft electrical supply, so if the Alternator and battery are not working the engine will still run.