The Clean Turnover: Spin the Bar
Greg Everett

The barbell has bearings for a reason—so it can spin. The turnover of the clean is the opposite of a curl—in a curl, the bar pivots around the elbow; in the clean, the elbow pivots around the bar. If the bar is not moved actively into proximity to the shoulders before the elbows are moved around the bar, the turnover will be slow, inaccurate and usually cause a crashing bar in a lower than necessary initial receiving position.
Pull the elbows up and out to move down under the bar, bringing the bar as high and back toward the shoulders as you can before spinning your elbows around the bar. Don't just hump the bar, pull your elbows back, and hope you can stand up after the weight falls onto your shoulders.

See the articles below for more detail on the turnover.