This mainly uses the muscles in your lats (the side of your back), but also engages your biceps and shoulders.

Clip the wide bar onto the cable machine and place it above your head, so you can reach it with your finger tips.
Place the pin on your desired weight for the exercise.
From a sitting, kneeling or standing position, facing the machine, place your hands wide on the bar. Your pinky finger should sit around where the curve begins, with an overhand grip.
Roll your shoulders back and down to engage your lats.
Pull the bar down towards your chest, squeezing your lats as it lowers in front of your face.
Slowly extend your arms back up.

Strength training: how to use a cable machine to get stronger