Grip the bar with your palms facing you, hands a little inside shoulder-width, and pull it down toward your upper chest while driving your elbows down and back rather than out to the sides. Lean back slightly as the bar comes down, then return to the start with your arms fully extended and your shoulder blades stretched up, rather than stopping short. The underhand grip puts your biceps in a stronger position to assist, so you'll likely feel more forearm and bicep involvement than with an overhand grip, alongside your lats. Squeeze your shoulder blades together at the bottom of the pull before letting the weight rise back up under control.