Anti-Lag systems or the Rally Bang Bang system do the following :
When the throttle is closed, an idle speed control valve opens up right next to the throttle body. The extra air is let into the intake manifold, and a bunch of fuel is dumped into the cylinders.
When the ECU detects that the throttle is closed, it instantly retards the timing to a point sooo retarted.. that the mixture is ignited WHILE the exhuast valve is still open.
This results in an explosion which forces the turbocharger to still spin.