Anti Lag is a system used in highly tuned turbocharged vehicles, to reduce the effect of turbo lag.
How a turbocharger works
A turbocharger works by exploiting the pressure of the exhaust gasses moving out of an internal combustion engine. It consists of a turbine - driven by the exhaust gasses - which is connected to a compressor.
The compressor forces air into the inlet manifold (hence forced induction). The higher density of the compressed air means that more fuel can be injected into the engine, which leads to much higher power output compared to a non-forced-induction car (also known as a 'naturally aspirated' engine)
In order to get up to full power, a turbo has to spool up - this usually happens when you get up in the mod-to-high rev range of the engine. Some engines use two smaller turbos ('twin turbo'), to give boost faster, but eliminating the lag between giving full throttle, and the power actually being available, is still a problem.
On high-powered cars, turbo lag can actually be dangerous in the hands of unexperienced drivers - the sudden availability of extra power can cause a car to lose traction and - in the worst-case situation - spin off the road and crash.
Anti-lag is a clever little piece of software built into an engine ECU. At low revs - when there is no turbo boost - the spark timing is held back for as long as possible. Instead of producing as much power as possible, this results in the combustion happening largely in the exhaust manifold. This causes the exhaust manifold to be hotter, but much more pressure is present in the manifold, which means that the turbos spool up, and that the power is available when needed. To keep the engine running in anti-lag mode, more air is needed as well, which is done by a throttle bypass valve or a throttle stop solenoid.
When Anti-lag is engaged, it often causes large flames to jet out of the exhaust system, along with pops and explosions in the exhaust system, commonly seen in world rally car (WRX) series car as they are off throttle.
Anti-lag is terribly bad for your turbocharger, exhaust manifold and exhaust system in general: the exhaust gas is significantly warmer (technically, still burning) as it is going through the turbo. The system will usually have an off switch. Cars fitted with the system need frequent check-ups on the turbos and exhaust systems.
So how much is the ignition changed?
rootbeer 277 raises an interesting question: "timing is held back for as long as possible - out of curiosity, what time scale are we talking about here? Microseconds? Milliseconds?".
I'm not actually sure, but I imagine the timing can be pushed back about an eighth of a revolution of the camshaft. For the sake of argument, let's guess this happens at 2000 rpm, which is 33 revolutions per second, which means that a revolution takes a 33th of a second. This rough calculation means that the retardation is around 264th of a second - that's about 3.78 milliseconds difference.