Actually, if you're trying to dip into the realm of reality with your explanation, the term you're looking for is 'afterfire'. 'Backfire' would be when the ignition charge ends up flowing through the intake instead of the exhaust. It's very dramatic, has a tendency to destroy intakes, and is in general a very bad thing. Afterfire is when unburnt fuel escapes the combustion chamber during the exhaust cycle and then ignites in the hot exhaust, throwing flames out the back. The reason this happens more with modified turbocharged cars is because they tend to run temporarily over-rich when you lift off the throttle from full boost. This is because the throttle plate has snapped shut (no more air), and the boost pressure is ported to atmosphere. Because boost air was already metered by the mass airflow sensor, it already had fuel allocated to it. Now that air isn't there, but the fuel has been injected into the engine. So out the exhaust it goes, causing the flames.

Close but no cigar, of course. Anti-lag is indeed the name for a method used primarily in rally cars to keep the turbo spooled up while off-throttle. The actual method varies, but in general the idea is to get combustion to happen inside the exhaust manifold instead of where it's supposed to take place, namely the combustion chamber. So either you divert air into the exhaust manifold when the throttle plate closes via a bypass valve and run pig-rich to dump fuel into the exhaust (thus causing the hot exhaust to become a combustion chamber), or you delay ignition timing so much that it happens practically on the exhaust stroke, forcing the hot combusion gases not to provide force against the piston but rather to be forced into the exhaust manifold.

The end result of either method is that hot combustion gases continue spooling the turbocharger hanging off the exhaust manifold while the throttle is closed (i.e. you aren't pressing the gas to spool it normally). Downside of either system is that an exhaust manifold isn't meant to be a combustion chamber and neither is your turbo, so this isn't a reliable modification for a street car. Rally teams don't care because they replace components anyway (and have lots of money).