This is a great exercise to work the Serratus Anterior while also working on the proprioception of the shoulder and encouraging increased rotator cuff activation. This exercise is a progression of the 1 arm Scapular Press against the wall as it takes away the stability of the wall.
Demonstration and Instructions
Place your hand on a ball against the wall, slightly below shoulder height (about 80 degrees flexion). Make sure your neck is "long" and chin tucked in and the inside of your elbow is facing up (shoulder joint externally rotated). Begin the movement by letting your body move towards the wall and your shoulder blade backwards towards your spine. Don't let your shoulder pop upwards. Then push your hand into the ball keeping it stable and push your body backwards so your scapular slides forwards around the ribcage, but do not allow the upper back (thoracic spine) to round, or the shoulder to pop up towards your ear.
Special Notes
Keep the elbow straight throughout the whole movement. The shoulder should not slide up towards your ear, it should only slide forwards and backwards around the rib cage.