I second what Smarthawk wrote
I "third" it.
The "dead cut" mag check technique was taught by the RAF on their Lycoming engined aircraft, which had a single rotary type magneto switch. This type of switch has three earthing contact positions in one switch, rather than two switches with an independent earthing contact in each. Two positions earth out one mag each, the "OFF" position earths them both.
Normal shutdown was from idle "Drop no stop, drop no stop, drop and stop, back to both" Then the fuel cutoff knob was operated to stop the engine from idle.
If a rotary mag switch has failed internally at the "OFF" position (i.e. the earth contact isn't earthing) and you stop the engine using the fuel cut off, you wouldn't know that one or both mags had been left in the ungrounded condition. So, the rotary switch points at "OFF" but
both mags could be live.
At idle, the "shock" caused by switching the mags back on again is no worse than that caused by a partially fouled plug shorting out during taxying or during a power check, After all, only one cylinder fires at a time.