i think this indexing is bull****,

but in aircraft engines it is recommended to alternate the position of the plugs to get better plug life, this should be done each 100 hours when the plugs come out for cleaning, gapping and testing.[it alternates the wear on the plug from the reversal of the direction of the spark].
in a car engine you would just throw in new plugs.
the average aircraft plug now is about $45-50 and the iridium plugs over $100, so it is worth the effort to look after them