The purpose of this movement is to work the muscles of your back, but will also work the bicep muscle.
Attach a v-bar handle or a straight bar attachment to a lat pull down machine. If using the v-bar handle, grasp it with a neutral grip or palms facing together. Next, sit down into the machine and secure your knees under the pads. Your arms should be fully extended over your body to start. Next, pull the handle to your upper chest; try to squeeze your elbows together behind your back. Pause the handle at the bottom of the movement and then return it slowly to the starting position. This will force you to squeeze your back muscles. Your torso should remain straight throughout the entire movement.
Squeeze the handle hard and it will work your forearms and also make the weight feel lighter when you begin the movement.
If using a straight bar attachment, duplicate the movement with the only difference being that you should use a grip with your palms facing your body or facing away from your body.