On my particular backfire I was lucky as there was no fire afterwards. Just a huge orange flash that went away, but it sounded like someone shot a .45 in the car! Like I said though, I must have sprayed 150+ times before it did this--and to this day I don't understand fully why it did.
In any case, engage the nitrous in 1st, but when you are over 3,000 rpm, and you should be fine. If I had an aftermarket converter that flashed to 3500 rpm, I don't think that backfire would have happened.
waldogt07---if you have a nearby source for 100 octane---use this when you know you are going to the track for 1/4 mile runs---trust me on this, its cheap insurance---the stock 3V piston has a very thin ring land to the top ring, its easy to damage. By putting in 100 octane your safety margin goes way up. Keep an eye on the plugs and bring extras (if you are running 125+). If there are problems, it will show up on the plug first.