NHRA uses a handicap starting system to equalize competition in certain categories. In essence, this system enables vehicles of varying performance potentials to compete on an equal basis. The anticipated elapsed times for each vehicle are compared, and the slower of the two cars is given a handicap head start equal to the difference of the two e.t.s. By using this system, virtually any two vehicles can be paired in a competitive drag race.
At NHRA national events, a handicap system is used in Competition eliminator, where the handicap is determined by national indexes, and Super Stock and Stock, where drivers are allowed to "dial-under" the national index, or select an elapsed time quicker than the national index. A driver selects an e.t., or "dial-under," that he or she thinks the car will run.
Here's how it works. If car A chooses a dial of 16.00 and car B chooses a dial of 14.50, car A will get a 1.5-second head start. If both vehicles cover the quarter-mile in exactly the predetermined elapsed time, the win will go to the driver with the best reaction time, or whoever reacts quickest to the green "go" signal on the Christmas Tree.
If a driver runs quicker than his or her dial, he or she is said to break out and is disqualified. If both drivers run quicker than their dials, the win goes to the driver who breaks out by the least. A foul start, or red-light, takes precedent over a breakout, so a driver who red-lights is automatically disqualified even if his or her opponent breaks out.