This exercise works the deltoids, in the shoulder, especially the rear head of each deltoid. At the end of the movement, when you pinch your scapulae (shoulder blades) together, you can emphasize trapezius and the rhomboids (in the back).
STEP 1. Kneel down on your hands and knees side-on to the cable machine, making sure that you keep your back straight and your abdominal muscles pulled in. Reach through and grab the handle, then take up the stack on the weight stack.
STEP 2. Keeping your elbow slightly bent, pull your arm back through until your upper arm is in line with your shoulder. At the same time, extend your lower arm and contract the rear of your shoulder.
STEP 3. Keeping the movement under control, guide the cable back to the start position, keeping the tension on throughout, and repeat for the desired number of reps.