Please DO NOT put all your faith in the Mag Switches being OFF.
Decades ago I spent many years as ground crew in the RAF on all sorts of propellor aircraft.
One of the tasks each morning was to turn the engines over using the prop to check for oil leaking into the cylinders (if it did the resulting hydraulic effect on startup could blow the cylinder head off).
We clearly always checked that the Mag Switches were OFF, but still had a fair few engines burst into life in the hangar........certainly catches your attention!
The cause was faulty Mag Switches or associated wiring.
Magneto switch circuitry is different to most other electrical stuff in that the electrical circuit has to be in perfect condition and the switches closed (ie technically "ON") in order to disable the Magneto.
SO IT IS NOT FAIL SAFE - if anything is broken or damaged THE MAG IS ON.
.............Never walk through the arc of a propellor, let alone touch it.
If you do have to turn a prop over, always assume its going to fire and move away from its arc.