I wouldn't know why youd want to use this in drag racing, just rev the engine and dump the clutch :shrug
I've seen this normally used in a rally racing and the idea is as follows;
Race into a turn at full boost, let off the accel. to brake going into the turn, the ECU senses the decel (lower TPS reading) and dumps fuel while pulling timing, you hear a loud popping that sounds like your exhaust manifold is going to explode (cause the fuel is detonating against the super-heated turbine wheel) and notice your boost guage hasn't dropped at all, come out of the turn full boost roarin' like a mutha and lookin' like a hero (well, the hero part is more about whether or not you can pull it off) savvy?