spark plug indexing

what side does the ground strap need to be on the spark plug for best performance ?

The general aim with indexing spark plugs is to align the ground strap so that the open plug gap is facing towards the intake valves and the incoming fuel/air charge. The theory is that this makes it easier to ignite the charge. I've tried this on our drag engines and to be honest never found any measurable advantage but then I guess we had sufficient spark energy to light off the charge anyway.