[Moppie]

There are lots of differnt anti-lag systems in use, and in theory.

The most common, seen on WRC and old Group A rally cars is a fuel injector and spark plug in the exhaust, it works just like unclebob descirbed it.
Its not very sophisticated, an you deffinitly can't use it on a street car.

However, Mitisubishi does have a more advanced system that is fitted to the Lancer EVO 7 and up non-US versions.
Im totaly sure how it works, but my understanding is when you lift off it open s valve system in the exhaust manifold, and some of the gas being expelled by the tubro is re-directed back into it with enough velocity to help keep it spinning for an extra second or two.

There are also electric assist systems being developed that attach a very high speed electric motor to the turbo shaft.