Side gapping spark plugs has been use in racing for years to increase horsepower by unshrouding the spark thus allowing the flame created to propagate faster resulting in a more complete burning of the fuel/air mix. Many manufacturers offer plugs deemed racing plugs that are side gapped but at inflated costs. By doing it yourself you can use a cheap .99 cent plug to save money and increase horsepower.
Using a dremel with a cutoff wheel or a hacksaw you will need to cut the ground electrode even with the edge of the center electrode. You need to be careful not to hit the center electrode or the porcelain.