Basically your trying to make enough gas in the exhaust to spool the turbo into boost so there is minimal lag when you set off.

Put it simply, the ECU turns the ignition timing about 45 degrees back, so it sparks with the exhaust valves slightly open, then the ECU squirts more fuel into the engine for a bigger bang, the engine fires with the exhaust valves open, causing the turbo to spool up. Which is also why you get fuel igniting in the exhaust