Originally Posted by Wikipedia

Anti-lag, (Sometimes called a Misfiring System in Japan, and a Bang-bang in Europe, due to the popping sounds made when the system is active), is a system used on WRC rally cars to completely eliminate turbo lag. It is a feature of the engine management system. WRC cars are fitted with a class mandated restrictor on the turbocharger inlet and this increases turbo lag significantly. Because of the pressure drop across the restriction, the pressure ratio for a given boost level is much higher and the turbocharger must spin a lot faster to produce the same boost as before.

Anti-lag effectively increases the mass of exhaust fed to the turbocharger. This is typically done by retarding spark timing to a point where most of the combustion happens in the exhaust manifold, typically around 40° ATDC. At the same time more air must be added, this can either be done by the use of a throttle bypass valve or a throttle stop solenoid. When anti-lag is switched on it is typically active when the throttle is closed, and when the throttle is opened the spark timing is resumed to normal.

Exhaust temperatures produced by this system are phenomenal but they can be controlled by varying the amount of fuel injected while anti lag is active; a richer mixture lowers the temperature. Anti-lag is typically only used on race cars because it can shorten the turbocharger service life down to impractical periods for street use. The turbine blade life is drastically reduced, and the entire exhaust system is put into shock, which can lead to explosion or fire. Rally cars generally change turbochargers every chance they get to do so.

http://en.wikipedia.org/wiki/Anti-lag