You race in the class you are best suited for. Bracket racing provides a handicap to even out the racers. Racers use test and tune time, qualifying rounds etc to see what they are running. Before eliminations, the driver selects a "dial-in" to post on the window based on those test runs. This is your handicap.
The person that can run closest to his dial and have the best reaction time off the tree without a foul or "redlight" wins.
So example, I ran 13.80, 13.83, 13.82. For this example, I dial a 13.81. My opponent dials a 14.81. He will get a 1 second head start (or two lights). If we both run our dial in exactly, the racer that has the best reaction time will win. Example, my reaction time is .150 and opponent is .100. I would have lost by .050 seconds. If you run faster than your dial-in, you breakout and lose, unless your opponent breaks out by more time or redlights.
To stage properly, turn on the top yellow light(small one of two) which is refered to as the pre-stage light. Then after your opponent is pre-staged, roll forward to turn on the 2nd light (staged) and after your opponent does the same, then the starter will start the tree.
That should get you started.