This "dead cut" several have mentioned presumably means they go to off with the magnetos. Fine, if you intend to let the engine come to a complete stop, and then wish to restart. However, it tells you nothing and accomplishes nothing. You already know the mags are grounding out by doing a standard mag check: both-left-both-right-both. No need to go to off. Off doesn't tell you anything that the other positions haven't already told you; the P lead is intact and grounding the mag through the switch.
This is not true. If you've ever had a rotary type mag switch apart you will know that the OFF position is internally a different terminal for the contact to 'make'. So, testing L & R is not testing the OFF position within the switch. And yes I have seen this problem and that was reason I took the switch apart. This may be the reason why the DEAD cut check appears in some check-lists.