Index racing is not heads up. In a heads up race, according to NHRA, "both cars leave the starting line at the same time, and the first to cross the finish line wins. Top Fuel, Funny Car, Pro Stock, Pro Stock Motorcycle, Top Alcohol Dragster, and Top Alcohol Funny Car race heads-up."

Further, they say "In Handicap racing, the object of the game is to predict how many seconds it will take your car to get to the finish line, then try to run as close to that number as possible without going quicker, or "breaking out." The driver who comes closest is the winner. Handicap racing allows cars of different speeds to race one other because the slower car gets a head start. In some categories, the driver chooses his or her own handicap, or dial-in/dial-under. These are Super Stock, Stock, E.T. bracket, and Jr. Dragster classes. In other categories, the class handicap is predetermined and may not be changed. These classes are Comp, Super Comp, Super Gas, and Super Street. In Super Comp, Super Gas, and Super Street, breakout rules apply."

http://www.nhra.com/streetlegal/dragracing101.aspx