when you switch to Left Mag you are grounding the Right Mag, that way the right mag is out of the picture and the engine is running on only the left mag. Why should a pilot care about this detail ? Well one good reason is when you are talking to the engineer he/she will know which mag to have a look at first.
The problem with the scenario I presented, is that there is no way to test the mag selected because selecting one mag still means both are providing juice to the spark plugs.
Aside from the obvious danger of having a live prop regardless of the mag switch position, you can not complete a proper mag check and so therefore the aircraft is not airworthy
Sorry for the thread drift
