Indexing from what I understand has nothing to do with the build. A very slight gain in the speed of the burn is accomplished by having the flat part of the electrode pointed at the intake valve so it fires as the gas mixture sprays into the combustion chamber. If the flat part is pointed away from the intake valve, the gas/ air mixture has to flow around the spark plug to get to the spark. The indexing is accomplished by marking the plug with a marker on the side that the flat electrode is pointing to. By knowing where the intake valve is located in each cylinder, the plug can be screwed in until it points in that direction. Each cylinder's plug will require different thickness shimmed washers to accomplish this. Some plugs may get tight right on the money, pointed at the valve. I have never tried this, figuring it was worthless unless you were wanting every horsepower that is possible out of an engine. I haven't seen a comparison of indexed vs non-indexed on the dyno but there may be one on Youtube.