It's my observation that many novice and intermediate weightlifters and crossfitters I see lack this ability. One of the most common mistakes that lifters can make is to rely on the speed of the upward moving bar and then drop under the bar. I don't like the word drop. It implies, to me at least, that there is no active use of the body.
You must alway be acting on the bar. When you pull off the floor, you are actively driving your legs. When you are transitioning under the bar, you are actively pulling yourself down. Carl Miller wrote in his book, The Sport of Olympic-Style Weightlifting, that "if you pull or push yourself under the bar, you will get under the bar faster than if you just drop below it."
The active pull of the arms is something that must be trained and practiced almost on a daily basis. Below is a list of exercises you can do to practice the active pull with the arms:
1) Snatch/clean rows
2) Clean row + press
3) Hang muscle snatch with no legs
4) Hang muscle snatch from the power position
5) Hang muscle snatch from above the knee
6) Muscle snatch from blocks above the knee
7) Muscle snatch w/ hip contact
8) Muscle snatch w/ no contact