Drafting (or slipstreaming) is positioning yourself behind a car in front of you, generally on a long straight, to reduce the effect of drag and increasing your speed due to their slipstream.
To see this effect in action, witness the magic of the Shake N' Bake technique used by Ricky Bobby in the movie Talladega Nights.
Anyway, this achievement only requires you to do 20 of these. On any long straightaway while you're not in the lead, position yourself directly behind the car in front as much as you can. It takes a few seconds of this to earn a 'perfect' drafting score but it isn't too difficult. This won't work on the slower cars during chase or pass events as you'll zoom by them too quickly (or ram into the back of them).
Good luck and enjoy racing! :)
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From Ex6:
"I have a feeling you have to perfectly draft 20 different cars. I was grinding for this in the Indianapolis oval track, followed one car during 10 lap race and got tons of the Perfect Draft scores but no achievement, then in my last 3 lap race I got it. Checked status and it says 84 Perfect Draft Scores."