During the drive phase, some athletes will begin pressing up on the bar before finishing the drive. This often leads to a dropping of the elbows into a more military-press-like position.
|Bottom of dip - elbows up|
|Halfway through Drive phase - elbows dropped|
As a result of this elbow drop, the bar rolls down the chest, or is suspended above the chest in the weightlifter's hands, leading to a significant loss of power due to a separation of the bar and torso and a forward trajectory on the finish of the drive.
[Athlete letting elbows drop during drive phase of Jerk]
Notice above how far the bar travels in front of the lifter. With heavier weights, this effect becomes more and more pronounced. At maximal weights, this is likely to cause a missed lift.
During the jerk, the lifter should maintain their rack position up until the point the bar begins to leave their torso, after the drive from the legs is complete.
[Ilya Ilyin performing drive phase of Jerk]
Notice above how the elbows stay in their position until the bar has left his torso, only then do they begin pushing up on the bar to drive the athlete underneath.
Sometimes this issue can be addressed simply by bringing it to the athlete's attention and asking them to finish the leg drive before pushing up with the arms.
Cuing the athlete to keep their grip loose on the bar until after they've 'jumped' through the bar can help fix the problem without causing the athlete to over-think the lift and hesitate.
Having an athlete do an exaggerated pause between the leg drive and the press-under with light weight can also be a way to help the athlete understand what you're looking for. After a few reps, the pause can be lessened and lessened until the press-under occurs immediately after the bar leaves the torso.