Drag it out

Before each race (also known as a pass), each driver is allowed to perform a burnout, which heats the tires and lays rubber down at the beginning of the track, improving traction. Each driver then lines up (or stages) at the starting line. Informal drag races can be started by any means, including flag-waving and arm-dropping. These methods are more likely to be seen in an unorganized setting, being most popular with illegal street racing. Sanctioned races are started electronically, with a series of vertically-arranged lights known as a "Christmas tree" or just "tree". The Christmas tree consists of a column of lights for each driver/lane. In each column, the top two lights are small amber lights connected to light beams on the track, which when broken by the vehicle's front tire(s) indicate that the driver has pre-staged (approximately 7 inches (180 mm) from the starting line) and then staged (at the starting line).^[2]
Below the staging lights are three large amber lights, a green light, and a red light. When both drivers are staged, the tree is activated to start the race, which causes the three large amber lights to illuminate, followed by the green light. There are two standard light sequences: either the three amber lights flash simultaneously, followed .4 seconds later by the green light (a Pro tree), or the ambers light in sequence from top to bottom, .5 seconds apart, followed .5 seconds later by the green light (a Sportsman tree, or full tree). If the driver breaks the starting line beam before the green light illuminates, the red light for that driver's lane illuminates instead, indicating disqualification (unless a more serious violation occurs).
Some cars rely on traction bars to prevent the torque from twisting the axles under heavy acceleration,^[3] transferring the torque onto the whole body. Also, some cars rely on wheelie bars to keep the front end from lifting off the pavement and wasting energy that would otherwise propel the car forward. On front-wheel-drive cars, these are used not to prevent wheelstanding, but to pre-load more weight onto the front wheels, increasing traction.
Several measurements are taken for each race: reaction time, elapsed time, and speed. Reaction time is the time from the green light illuminating to the vehicle leaving the starting line. Elapsed time is the time from the vehicle leaving the starting line to crossing the finish line. Speed is measured through a speed trap near the finish line, indicating the approximate maximum speed of the vehicle during the run. Top Fuel Dragsters and Funny Cars are now running 310 mph (500 km/h) in a thousand-foot [1,000-foot (300 m)] race.
The winner is the first vehicle to cross the finish line (and therefore the driver with the lowest total reaction time + elapsed time) without breaking out (going faster than a dial-in) or redlighting. The elapsed time is a measure of performance only; it does not, per se, determine the winner. Because elapsed time does not include reaction time, a car with a faster elapsed time can actually lose. In practice, it is advantageous for the driver to launch the car during the split-second interval between when the yellow light goes out and the green light goes on. Once a driver commits a red-light foul, the other driver can also commit a foul start by leaving the line too early but still win, having left later. A driver who gets a substantial lead at the start is said to have gotten a "holeshot". A win where a driver wins a race with a higher elapsed time but lower reaction time is known as a "holeshot win".
It is also possible for a driver to be disqualified for other infractions, depending on the rules of the race, including crossing the centerline between lanes, touching a wall, striking a track fixture, failing to stage, failing a tech inspection, or running faster than expected/allowed for the assigned class. In boundary line violations, if the offending driver has made a clean start, and the red-light driver does not commit the violation unless forced by the offending car for safety reasons, the driver who committed a red-light foul wins.
In the common Eliminator racing format, the losing vehicle and driver are removed from the contest, while the winner goes on to race other winners, until only one is left. In cases where a driver has no opponent for a round, the driver makes a solo pass or "bye run" (in order to at least partially eliminate the advantage that would otherwise come from the engine having one less run on it) to advance to the next round. In most Eliminator formats, the bye runs take place only in the first round. On bye runs, some drivers may choose to drive slowly so as not to stress the car unduly, though choice of lane in the each round is often determined by time in the previous round, making this strategy possibly detrimental. Unlike the NHRA, many European events feature a consolation race where the losers of the semifinal rounds race for third place, the final spot on the podium, and standings points.
During drag racing events, vehicles are classified by various criteria that take into account the extent of modifications to the car. These criteria include engine capacity, configuration of cylinders, frame type, vehicle construction materials, wheelbase, horsepower (or displacement) to weight ratio, number of cylinders, whether or not power adding equipment (turbocharger, supercharger, or nitrous oxide) is employed, vehicle type (such as car, truck, or motorcycle), or even make and model for limited entry fields. The aforementioned divisions are in place to ensure cars are evenly matched during the race. Not all of these apply at once.)