A transmission brake is usually considered a drag racing "only" automotive after-market device. It operates by simultaneously applying the forward and reverse clutch packs of an automatic transmission while its both in gear, and the torque converter is at or near it's stall speed. At the moment the trans-brake is disengaged the reverse element is released (usually the engine is revving at peak power) and the car will immediately lunge forward from its starting position.
You might wonder what the difference is between holding back a revving car in gear with just the wheel brakes versus that with a trans-brake. In order to understand this difference, more must be explained about the car's suspension, and how it operates during the initial take-off.
If your car has a rear-axle drive with a solid suspension, it most likely also has what's called "anti-squat". This term defines the function of the rear suspension geometry that prevents the body of the car from "squatting" during it's initial acceleration. In fact, the rear of the car will rise if the anti-squat factor is high enough to overcome the inertial forces encountered during this initial acceleration. Many cars from Detroit incorporate an anti-squat design in order to prevent suspension "bottoming" during initial acceleration. You can find out if your car has this feature by performing a simple stall test and if the rear end rises, it has anti-squat. If your car has an independent rear suspension, such as the Corvette, it will squat because this type suspension has a "pro-squat" type geometry built into it.
How is the trans-brake and the rear suspension related? Keep in mind that for every force in one direction, there is an equal and opposite force in another. The force that is opposite to the raising of the rear will appear as high instant tire loading during initial acceleration. Its instantaneous because this force only occurs at a moment during the initial launch. This at the time between when the car is at rest and when full power is being transferred to the rear suspension.
Since the traction forces of the rear tires usually increase relative to their contact patch force, a car having more anti-squat will always have a higher acceleration and therefore will launch much quicker. If the wheel brakes are used for staging, the rear suspension becomes preloaded and all the anti-squat factor is already used up. When a trans-brake is used for staging, the rear suspension remains at rest and is unloaded. At the moment you flick the switch and release the trans-brake, the immediate transfer of engine torque "shocks" or loads the rear suspension, performing as it would with a stick-shift car. The trans-brake also prevents the high-powered/high stall car from overpowering its brakes on the line. By comparison, the trans-brake release action is much faster than lifting your foot off the brake pedal (or even a clutch pedal for that matter).
The trans-brake is used in conjunction with a specially calibrated full manual valve body. During operation the transmission will function normally, even if you choose not to use the trans-brake. A toggle switch is used to simply turn off its operation. Since the trans-brake is an added feature, the operation of the transmission can not be adversely affected when it is not being used.
The FB trans-brake is available for the following types of high performance transmissions:
GM - TH350 /TH400 / Powerglide
FORD - C-4 / C-6 / 4STB / AO3
MOPAR - TF727