Big Mistake: Not Taking the Slack Out of the Bar
If a lifter doesn't pull the bar up so that it's making contact with the top of the rim in the hole of the plates, he's initially not going to be working against any resistance. By not working against any resistance, it's extremely difficult to produce an isometric contraction in the spinal erector musculature and the lats that's strong enough to lock the lumbar spine into extension during the movement.
It takes 0.4 to 0.5 seconds for the average person to generate maximal muscular tension (0.25 seconds for well-trained athletes), so if the lifter attempts to "jerk" the weight off the floor as quickly as possible from a relaxed position, he's not providing enough time to produce the necessary lower back stiffness. The end result is needless lumbar flexion as soon as the bar begins its journey against gravity, which can then lead to complaints of "hurty back syndrome."
Take the Slack Out
Once you've set up properly, the final thing you should do before liftoff is to pull the chest up as high as possible, making sure you simultaneously pull the bar up as hard as you can against the weights. (See video.)
On some plates, there's a small gap between the bar collar and the top rim of the hole – listen for the telltale "clink" as they make contact and then hold this position for a brief moment before lift-off.