The reason why there are differences is that, there is a general lack of understanding of how the magneto switch is wired. In the case of most puddlejumpers the magnetos are selected by a rotary switch, usually controlled by a key. In a few types (e.g. Chipmunk, Seneca) they are selected individually by toggle or rocker switches.
In the latter case, a check of each magneto switch in turn will confirm, by a drop in RPM, that the appropriate magneto is earthed when the switch is selected off. Nothing is gained by selecting both switches off together. You know that both magnetos will be safe.
However, in the case of the rotary switch, confirming an RPM drop at each of the 'L' and 'R' positions does not prove that the magnetos will be earthed when the switch is set to the 'OFF' position. On the back of the switch are connected three earthing wires, one earths the right magneto when 'L' is selected, one earths the left magneto when 'R' is selected and the third earths both magnetos when 'OFF' is selected. If the 'OFF' wire is disconnected or broken then a pre-shutdown check of 'L' and 'R' will indicate that each magneto can be switched off but, when the switch is selected to 'OFF', both magnetos will be live.
It must be remembered that a check of the 'OFF' position of the rotary switch must be carried out ONLY with the throttle fully closed to avoid unnecessary strain being imposed on the cylinders and crankshaft.
You can see that, contrary to Turbine's post, both checks do not accomplish exactly the same task. One confirms that the magnetos will both be earthed when the rotary switch is selected 'OFF', the other doesn't. Also, remember that, even if a 'dead cut' check is carried out, the system is fail-unsafe and the prop must always be treated as live. Think about that the next time you are checking the alternator belt - now there's a pointless and dangerous check!
[This message has been edited by rolling circle (edited 18 January 2000).]