You need to press the accelerator button at the end of each count. Three(press), two(press), one(press), GO!. If you get the timing right, you'll get a flare at the back of your vehicle that changes color with each correct press. If you mess up, it'll sputter out in a puff of smoke, but you might be able to still get a partial boost if the count's not done. Done entirely right, you can get a stage 3 boost off the starting line.