The reason for this item is that a spark plug may produce a
visible spark indicating there is enough voltage to span
that gap.  There may not be enough voltage to span the gap
inside a presurized cylinder with a fuel charge.  With the
adjustable gap device, you can adjust to a gap specified by
the manufacturer.  If you get a nice clean blue spark at
that gap, then you probably have enough spark to get the job
done.  The spark plug "looking" tester is what I use on
small engines.  I use the adjustable one with a larger gap
for a quick check on HEI automotive units because they
require a bit more spark.  This is only an indicator if you
have enough spark to start an engine. It may not be good
enough for best performance.  That may take a bit more
diagnosis

Lugnut.