The idea is to get the incoming charge spraying into the spark gap instead of "finding its way around the ground electrode". I don't know why it works, but it does a little. The idea is to get the open gap pointed toward the intakes. On V-8 engines you make a mark on the same side of the insulator as the ground electrode. Spin them in finger tight and see where the mark is. Since finger tight + 1/4 turn is a good tightening practice, make sure the mark is 1/4 turn away from the exhaust side of the cylinder. You keep moving the plugs from cylinder to cylinder until you get a good enough alignment, or you get a couple more plugs and try again. As mentioned, you can get indexing washers, but I've never used them.
I suspect that some of the improvement is due to the incoming charge both cleaning and cooling the electrodes.