Anti lag works on closed throttle situations, (aggressive ALS will run a trailing throttle ALS though)

It works by a valve bypassing air from one side of the throttle body to the other, allowing the engine to rev up as if the throttle was open. The ecu will then cut fuel and ignition in a sequence of which will allow the engine to run but also to ignite excess fuel whilst passing through the engine and ignited in the exhaust manifold. These explosions in the exhaust manifold cause the exhaust wheel of the turbo to be turned, each explosion will obviously work in turn and drive the exhaust wheel hard on each bang.

The more air bypass the more aggressive the ALS can be.