Creampuff, do you get your little wisdoms from Savvy or the like.
Perhaps this procedure they advocate is what you're on about.
In-Flight Lean Mag Check
The in-flight lean mag check is a test of ignition system performance, and is used to help diagnose problems with magnetos, ignition harnesses, spark plugs, ignition timing, etc.
Perform the in-flight lean mag check procedure by setting up the airplane in normal cruise on autopilot. Then perform the following procedure.
Lean to an aggressively lean cruise mixture. The leaner, the better for this test. The test will be much more discriminating and meaningful if you lean to a lean-of-peak-EGT (LOP) mixture setting. Preferably lean as far LOP as you can without experiencing significant engine roughness.
For a key or rotary style magneto switch (as found in most singles), select BOTH-LEFT- BOTH-RIGHT-BOTH, leaving the mag switch in each of these positions for one full minute.
For individual magneto toggle switches (as found in most twins), turn off the LEFT ENGINE/LEFT MAGNETO switch for one full minute, then turn it back on and wait for one full minute. Repeat this procedure with each of the other three magneto switches in sequence. (LE/LM, LE/RM, RE/LM, RE/RM.)
NOTE: It is normal for engines to run a bit rougher on one magneto than they do on two, but they should not run change-of-underwear rough. Please report on the level of perceived roughness during phase of the test.
NOTE: For turbocharged engines, TIT may rise to above red-line during single-magneto operation. This is normal and will not do any harm if the exceedences do not exceed a few minutes’ duration.
Not too sure if the 'Note' at the bottom of that procedure would gain the Engine OEM's agreement. Perhaps the whole procedure is flawed. I'm not educated sufficiently enough to know. Are you?