Cable pullovers allow for a great stretch and full contraction of the lats. The added benefit of having your entire back supported by lying on the ground lets you focus on volume without the fatigue accrued by supporting yourself.
Remember, big lifts first, then do this low-stress finisher for added volume.
So you can squat 405 pounds and bench 315. Is that good? Does that make you strong? How do you compare to others? Find out if your lifts are decent, good, or great through Tim Henriques' handy little comparison chart.