Stand facing a high cable with a bar or rope attached, arms extended down in front of you with a slight bend at the elbow that you hold fixed the entire time. Pull the bar down in an arc toward your thighs by driving your shoulders down and back, not by bending your elbows -- your arms are just the connection between the bar and your shoulder blades here. Let it return under control back to roughly eye level, keeping that same slight elbow bend throughout rather than letting your arms straighten and re-bend on every rep. Because your elbows stay locked in place, this isolates your lats without your biceps jumping in to help the way they do on a normal pulldown.