I've never heard that. Both plugs fire simultaneously with a Boyer (wasted spark). If you foul a plug and it quits sparking, does the black box sustain damage? My idea behind this "test" is; you have a good-running engine, but a gauge tells you only one cylinder is up to snuff, so the good one should idle stronger on it's own than the weak one. If it does, that confirms the compression tester's reading. If both idle the same, something is amiss in the gauge or the operator. Truthfully, a leak-down test is the best way to assess the compression.