easy, you hit the brakes to build boost w/o moving, honestly with a stock turbo on an sti its not necessary, this is done on automatic supras and whatnot with big turbos from a roll for no lag. A lot of drag cars use whats called anti-lag from a dig to have boost from a stop, this is basically a lower rev limiter that the car bounces off

 

So if he's doing that in that video, how could he stay at the same speed as the car next to him and be boosting that much before they even start the race?

 

just balance the gas and the brake. It's really pretty easy and fundamentally one of the same benefits a circuit course or autoXer gets from left foot braking (improved turbo response for acceleration)