I could not get this through natural play.
You unlock this achievement during a head-2-head race. Career and quick play both count. You see your reaction time on the upper right corner of the screen after the race starts. You can also check your reaction time on the result screen at the end of the race.
There's the burnout, bump and then countdown to the start of the drag race. Only the last part counts. You can take your time with the burnout and bump. It doesn't matter. I let go of the brake after seeing the "Go!" and always had a reaction time of .275 or worse.
The only way I got this was with luck. I let go of the brake and hoped that it was not a false start. It took me a few dozen tries. And you have to sit through all the screens, burnout and bump again, which can get pretty annoying real fast.
I got really lucky: