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.