took a bit but I found some info thanks. Big in the rally world.
"Anti lag was devised to keep the turbo spinning in a throttle off position. A very late ignition spark (~30° retart and overfuelling causes the flame front to exit the engine through the open exhaust port.-- Read more at:
https://www.torquecars.com/tuning/anti-lag-systems.php - (c) TorqueCars"