Technique advise on squat cleans from Bill Starr. This article was published in the CF Journal in 2009.
Most beginners fail to fully extend before moving to the bottom, fearful that they aren’t going to get there in time to rack the weight. But because they don’t fully extend, they don’t provide that final, critical jolt to the bar, and that jolt is needed to allow them time to jump into the bottom. In addition, when the pull is cut short, the body is leaning forward slightly, and this is not what you want. It costs time for you to return to the upright position. Otherwise, you’ll be leaning forward when you rack the bar, and this usually results in the bar being bumped.
Knowing when to bring the traps into play and when to bend the arms are the two most difficult parts of the pull. Bring the bar off the floor smoothly in a tight, controlled line. When the bar passed your knees, drive your hips forward forcefully and immediately contract your traps. All the while, your elbows are locked and arms are straight. But once you activate your traps, bend your arms and drive your elbows up and out to provide that final bit of impetus to the upward-moving bar. At that same instant, you should be high on your toes. This latter point is extremely important for a couple of reasons.
The calves help to elevate the bar a bit higher, and quite often that bit is the difference between success and failure. You can also move to the bottom faster when you’re on your toes than when you’re flat-footed. When your body is perfectly vertical with your elbows up and out and you feel the bar jump, that’s your cue to drive to the bottom.
These paragraphs really touch on how to get the weight going up, and how to pull under it quickly for the catch.