John Stevens wrote:

Seems to me (a rank novice, almost) that the RPM drop is the
indication of actually switching off one of the mags to allow you to
test the other in isolation.

It usually is, but if the timing on both mags is retarded a little (about 10 degrees
does it on my O-320), the engine will run at the same speed on either or both mags.
This could be the problem, or, as others have said, the switch could be failing to
ground the mags in the "L" and "R" positions.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.