Before you can earn this, you obviously have to earn a bar of boost (green segmented bar) by drifting around earlier turns.
Once you have a full bar, drift
into any turn, and about half-way (or 3/4) through the curve, hit
to boost. You need to boost during the drifting
, and then maintain that same boost after you stop drifting
when you straighten out. Achievement should pop right after you finish boosting.