The best cue is to keep your shoulder blades back and down. The first few reps show the incorrect position. In the last few reps, the shoulder are back and down.
For any horizontal pulling exercise, like seated cable rows, keeping the shoulders back and down will ensure your lats and mid back are being hit the hardest, instead of just the upper traps and biceps.
In the video, the first couple of reps are performed incorrectly and the last couple of reps are performed with the shoulders back and down.
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Do this exercise before a chest workout to activate the pecs, or after a chest workout to really finish it off. Squeeze as hard as possible for 20 seconds per set.