An old racers trick that indexes the opening of the ground electrode towards the intake valve for a better exposure. Does little on the street bikes with stock motors and is most effective on high compression motors where the flame front is very critical to a good burn.

There are so many different views on how to properly index a plug that the benefits no matter how small get lost in the interpretation.

Some input on it here.

Indexing Spark Plugs (http://www.nightrider.com/biketech/hdsparkplugs_indexing.htm)

Indexing Spark Plugs | Advance Auto Parts (http://www.advanceautoparts.com/english/youcan/asp/pht/pht20001001sp.asp)