A transbrake is a device used by drag racers to launch their race cars in an extremely powerful manner. Installed within the valve body of an automatic transmission, the transbrake is an electronically activated device that effectively locks a transmission in both reverse and low gear at the same time. With the transbrake activated, the driver can bring the engine up to a very high speed while it is sitting still. When the starting light turns green — or a fraction of a second prior to it turning green — the driver releases the transbrake, which causes the transmission to snap into gear and send the car down the track in a very powerful manner. Often, the release of the transbrake is accompanied by a huge wheel stand, or wheelie, as the engine sends all of its power to the rear wheels at once.
Releasing a transbrake on an automatic transmission is tantamount to dumping the clutch on a stick shift vehicle while the throttle is being pressed to the floorboard. The transmission as well as the entire drive line must be heavily modified and reinforced in order to operate a transbrake without destroying the vehicle. Many times a rear end will break or an axle will snap as the power is released in such a violent manner. Drive shafts have even been known to twist in two as the rear tires grip the surface of the track and refuse to slip and spin.
While many transbrakes are released manually by the driver, the vast majority of them are connected to a device known as a delay box. This electronic device is able to be programed to release the transmission at a set time. Typically set to correspond with a driver's individual reaction time to the starting light, the delay box — as its name implies — delays the release of the transmission to correspond with the flash of the starting light. The closer a driver can launch his race car to the illumination of the green starting light, the better the reaction time and the faster the car will reach the finish line.
In an effort to avoid revving an engine too high for too long, a device known as a two step is also typically wired into the transbrake and delay box. This device holds the engine at a reduced rate of speed until the transbrake is released. Once released, the two step box allows the engine to reach full speed instantly.