Lie supine (on your back) holding the pulleys’ stirrups (handles). The stirrup from the right pulley should be in your left hand and the stirrup from the left pulley should be in your right hand. The cables should be crossed and pulled taut.
Keeping your arms perpendicular to your torso and your elbows slightly bent, exhale as you pull your arms open and out to the sides.
Hold for a count of two.
Inhale as you return your arms to the starting position.
Comments and tips
Keep the movement slow and deliberate.
When you lie down, make sure that your shoulders are lined up with the pulleys. This will ensure optimal mechanical leverage.
By keeping your arms perpendicular to your torso, you will minimize the involvement of your latissimus dorsi and keep the tension on your posterior deltoid.
Instead of lying on your back (supine), you can lie prone (on your front) and use two low pulleys. This exercise is known as the lying cable reverse fly. See the second video.
The supine cable reverse fly is also known as the supine cable rear delt fly.