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.
For this upper-body warm-up, perform a barbell military press followed by an overhead shrug for 15 reps. Keep the arms locked. Focus on raising your shoulder girdle up and down.