Grab your pulldown bar and sit down on the lat pulldown machine. Extend your arms overhead and keep them straight throughout. Shrug your shoulders down and back. Imagine you are trying to put your shoulder blades in your back pockets. Let your shoulders rise up to your ears and repeat.

Lat shrugs are basically very short-range pulldowns. You pull entirely with your lats and your arms bend only at the "bottom" of the ROM (as the weight's closest to your body). Even then, the bend is very slight; you only get a little biceps action involved -- at least, the way I did them.