Stand sideways to a cable or resistance band with your feet hip-width apart and hold the cable/band handle. Engage your abs to brace your torso. Keeping your torso vertical to the floor, pull your shoulder blades down your back without arching your lower back.
Exhale and slowly rotate your torso away from the cable/band anchor, keeping your upper arms at your sides, elbows bent and hands positioned at the midline of your body. Your head, chest and torso should all move together. Avoid leaning your torso in the direction of your movement.
Inhale and rotate back to starting position. Repeat.
Variation: Repeat the same exercise, but fully extend your arms to shoulder height and keep your arms in this position throughout the exercise. The longer lever increases the loading on the spine, requiring the core muscles to work harder.