This exercise develops lats. It allows you to work out each arm separately, loading the lagging muscle group.
Sit down on a machine facing it (or otherwise, depending on the model).
If there is a back support, press your body against it firmly.
Grasp the handles.
Take a deep breath.
As you exhale, pull the bar down bringing the elbows close to the body at the same time.
As you inhale, return to the starting position.
Do not swing or try to pull the bar down fast.
Do not pull the bar down with the straight arms locked in the elbows.