Original question was "why quick mag check before shutdown"
DO it slow and you will hear why it is recommended to do it quick.I will type real slow as I know you will be reading this slow .
The mag check that is done before shutdown is a safety check to ensure the mags are NOT LIVE when the mag switche's are in the OFF position. By turning the mags off the engine will normally not get a spark.
If there is a broken P-lead the off postion will not ground out the mags and the engine will have live mags.A very dangerous situation as the engine can start by turning the prop. Even with the mixture out there can be enough vapour to ignite and hurt someone .