Bringing the bar to the neck with arms alongside the body mainly solicits the lateral fibers of the latissimus dorsi and develops the width of the back.
Bringing the bar to the chest by pulling the elbows back to raise the chin to the bar mainly solicits the upper and central fibers of the latissimus dorsi. This variation is excellent for developing the bulk of the back.
Images are taken from Strength Training Anatomy by Frederic Delavier.
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