The "dead cut" should not be confused with a mag check which is what Bose describes.
It is called a dead cut because you do indeed select the OFF position briefly. As Bose says this is not good for the engine if you do it at anything above idle and go back to L,R or BOTH causing a backfire etc
If you fly an aircraft with individual switches for the left and right mags then turning each to off and back to on in turn as you do on a mag check tells you that with both switches in the off position, both mags are grounded.
However, on the rotary switch, selecting left and right may give you a drop but this only checks the L and R positions you have not checked the OFF position with this check.
Far better to check the OFF position before the first flight of the day -
Start the engine and allow it time to warm a little. Check the L and R positions give you a mag drop (they are working). Then with the throttle closed, select the OFF position. Leave the switch off until the engine stops. You can then re-start the engine and be happy that the switch is mostly working as it should (you have not checked the start position).
Using ICO to shutdown as has been said earlier leavs the plugs dry etc etc.
However, it is no guarantee that the engine will not fire if you abuse the prop. How many have hopped into the old C172 and started the engine with the mixture ICO. It will fire and promptly stop but enough to hurt your parts that are in the prop arc.
Regards,
DFC