Part of the confusion here is the different switch conventions between the UK and North America.
In the UK, a normal single-pole switch is "down for on". In the US and Canada, it is "up for on". US aircraft follow the local convention. Here's a FS image of a panel I'm very familiar with - the Bellanca Citabria:
In this picture, the left three switches are Off and the other switches (Panel Lights, Master, Avionics and Mags) are all On.
As SSD points out, when the Mag switches are ON (up in this case), they are in fact open-circuit. When the Mags are OFF, the circuit is closed and the magneto primary coil is grounded, thus preventing it from generating any voltage.
Part of the shutdown checks is to run your hand along the row of switches to confirm they are all down.
I suspect that the reason that mag switches on British aircraft are also down for OFF (I'm thinking Tiger Moth - I'm not familiar with other types), goes back to WWI when domestic light switches were used for mag switches and they were installed in the conventional sense - down to close the circuit and thereby ground the mag.