Bang-bang (also known as ALS which stands for Anti-Lag System) is an engine management technique that allows to minimize the turbo lag time.

As you might be aware of, turbochargers display what is known as lag time which is the time needed for the turbine to reach its full throttle from an intermediate rotational speed state. The duration of a turbocharger's lag depends on many factors among which its inertia, airflow efficiency, back pressure, etc. The problem is partly dealt with by fitting a turbo dump valve, which acts each time the driver lifts his foot from the throttle. The dump valve will evacuate the pressurized air coming out of the turbocharger while the inlet manifold is closed thus allowing not to stall the turbine and avoiding possible damage to its bearings. In racecars it is very common to fit oversized turbochargers in order to be able to produce enough boost pressure and assure a sufficient engine output. Big turbochargers display significant amounts of lag due to their increased inertia. In these cases the dump valve is insufficient to allow the turbocharger not to loose too much speed when the driver lifts off. Additionally rally cars hold a turbo restrictor, which is regulated by the FIA. One of the restrictor' effects is to increase lag time. This is why in racing cars, and most specifically in rally cars where torque and engine availability are critical factors, most applications use anti-lag systems.

During lag time the engine is much less responsive and its output well below nominal. To counter the effect of turbo lag time drivers used to anticipate the engine's reactions by accelerating well before they would have in a non-turbo car. Others have used a technique, introduced by the German driver Walter Röhrl, known as "left foot braking" where the driver uses his left foot to brake the car while his right foot accelerates to keep the turbocharger in optimal load. ALS was a simple idea but relatively difficult to implement. To the best of my knowledge Toyota Team Europe were the first to use it in racing (Toyota's implementation is known as Toyota Combustion Control System while Mitsubishi call the system Post Combustion Control System).

How ALS works

While the ignition  timing is altered with 40° or more of delay the intake air and fuel supply is made richer when the driver lifts his foot from the gas pedal. This results in air and/fuel mixture that keeps getting in the combustion chambers when the driver no longer accelerates. The ignition being delayed, the air/fuel mixture reaches the exhaust tubes mostly unburned. The exhaust temperature being extremely high the unburned fuel explodes at the contact of the exhaust tubes. Luckily the turbo sits right there and the explosion keeps it turning (otherwise it would slow down since its intake, the exhaust gases, is cut-off). The effect is vastly lower response times with some downsides:

• A quick rise of the turbo's temperature (which jumps from ~800°C to the 1100°C+ region) whenever the system is activated
• A huge stress on the exhaust manifold and pipes (mounted on a street car a bang-bang system would destroy the exhaust system within 50-100 km)
• The explosions which occur in the exhaust tubes generate important flames which, sometimes, can be seen at the end of the exhaust tube

The the bang-bang system got its name from the loud explosion noises one hears whenever the driver lifts his right foot.  Depending on the implementation, ALS systems are more or less aggressive. While the systems mounted in Toyota and Mitsubishi racing cars are relatively smooth and noiseless those fitted in Ford and Subaru cars are much more noisy. Most racing implementations have user selectable anti-lag settings depending on the terrain, usually three settings can be selected by the driver going from moderate to very aggressive.

It's available in Astra Racing  to the general public an ALS - ECU for the Evo2.
Fits as the original ECU and, with a switch you can decide to activate the ALS. More info, contact Astra's site.