It's quite straightfoward, but requires you to be consistent across the laps. I have included the video description here as it explains what you need to do in order to get it.

To get this achievement, you need to have unlocked the FIA Rallycross championship by getting to Professional level in Rallycross career. Make sure you have a fully upgraded Rallycross supercar, preferrably with a decent tune, but the default should suffice. In any of the 3 events, progress to the final and, if possible, try to start at the front of the pack by winning the semi-final (although as you can see, this isn't necessary).

I would strongly recommend you do this on either Lydden Hill (England) or Holjes (Sweden) as these are the easiest and have the best joker laps. Personally I'd recommend Lydden Hill but Sweden is fine too. You want to jump the start JUST BEFORE the "2" appears in the countdown, otherwise you get disqualified immediately if you jump when the "3" is still on screen. Try to keep the speed in the joker laps, and they won't cost you much time-wise but will make jumping the start worth it. You need to get your timing on when to reverse right - too late and you'll miss it. Too early and you'll lose.

Note that you need to be accelerating (assuming you're using manual transmission) in reverse gear when you go for this. It's not enough to be going backwards when you cross the finish, as I tried earlier and didn't get it.