SSD:
On conventional Lycoming/Continental engines with rotary mag switches, you shut down the engine by moving the Mixture to ICO, then, when the engine has stopped, switch off the Mags. This not only prevents 'pre-ignition' ot 'running on' after switching off, but has the advantage that the engine is left in a doubly safe condition, as both the ignition is off, and the idle system is drained of fuel.
You carry out the 'Live Mag' check just before that, with the throttle closed, and the engine idling. The reason you put the mags back to 'on' is to enable the idle system to drain when you put the Mixture to ICO.
MJ
