HOW: Rotate your shoulder from internal to external rotation and repeat. When in external rotation, think about scooping back your shoulder blades while attempting to point your thumbs towards the floor. When internally rotation allow your shoulder blades to come up and forward while thinking about bringing your pinky up towards the ceiling.
FEEL: You will feel the shoulder and shoulder blade muscles working with this exercise.
COMPENSATION: Avoid arching your low back as you rotate your arms externally or backward.