Line locks are electrically-operated valves installed in a vehicle's brake line(s). They are used to hold a vehicle in place while the driver is concentrating on the application (sometimes violent) of power to the drivetrain.
Drag racers use them to help hold their position at the starting line until the last possible moment.
Offroaders use them to hold their position on a trail obstacle while changing gears or otherwise using one's feet for tasks besides applying the brakes.
line locks are similar in use to and offroad truck's emergency brake with several important differences:
Line locks are activated to hold the pressure applied to the brake system AFTER the operator has pressed on the brake pedal. Once the wheels are locked by the brakes through the use of a line lock, the only way to release the brakes is to release the line lock. The brake pedal is completely useless with the line lock engaged. If the line lock is engaged when there is no pressure on the brake pedal and the wheels are freely turning, the brake pedal is totally useless for purposes of stopping. The line lock, when engaged, blocks the flow of brake fluid from the master cylinder to the brakes so that no amount of pressing in the brake pedal will cause the truck to stop rolling until the line lock is released. As you can see, line locks, when used by an untrained operator can be VERY dangerous and are not a replacement for the factory emergency brake.
Once upon a time, there was a company called Pork Barrel Engineering that sold a line lock kit for the Suzuki Samurai. Unfortunately, they are out of business (or at least no longer have a web site promoting the produt anymore). Fortunately, there is an informative article at Off-Road.com that reviews Pork Barrel's line locker kit. See it at Product Review: Pork Barrel Engineering Line Lock Kit . How long it remains there, no one knows...
For more information on line locks for automotive braking systems visit Summit Racing and Jeg's High Performance