The ignition is retarded (when the driver lifts off the throttle), not advanced.
Here's one explanation:
http://www.rallycars.com/Cars/bangbang.html
When you suggest a seperate gas (petrol?) injection, I guess you mean in the exhaust, just before the turbo? I think this was tried/tested in 1980's F1 (Ferrari?) but possibly banned, because then the turbine could be considered a seperate "engine".
Street driven cars and cars driven in some competition events that don't allow anti-lag (Targa NZ tarmac rally is one that doesn't allow it) have to use other methods such as blow-off/dump valve to help keep the turbo spinning during off throttle conditions.