Starting Position: Grasp two dumbbells and lie supine (on your back) on a flat bench with your feet placed firmly on the floor or upon a raised platform to allow you to maintain a neutral (flat to normal) spine. If working with a spotter, signal the spotter for assistance in handing you the dumbbells.
Depress and retract your scapulae (pull shoulders down and back) to make firm contact with the bench. Your head, shoulders, butt and feet should make contact with the bench and floor/riser throughout the exercise. Signal the spotter for assistance in moving to the starting position. Grasp each dumbbell with a closed, pronated grip (palms facing forward, thumbs wrapped around the handle), pressing the dumbbells to a position shoulder-width apart with your elbows extended. Externally rotate the arms (rotate your elbows to face outward and palms to face inward), slightly flexing the elbows, but maintaining a neutral wrist position (avoid flexion/extension at the wrist joint).
Downward Phase: Inhale and slowly lower the dumbbells in unison in a wide arc until they lie level with your shoulders or chest. Keep the dumbbells parallel with each other during the movement. Maintain your slightly flexed elbows, neutral wrist position and contact of your head, shoulder, butt and feet with the bench and floor/riser.
Upward Phase: Exhale and slowly raise the dumbbells upward in a wide arc back to your starting position, keeping the elbows slightly flexed, wrists neutral and making contact with the head, shoulders, butt and feet with the bench and floor/riser.
Safety is very important in this exercise. A spotter should adopt a split-stance position or half-kneeling position with the hands positioned as close to the most distal (wrist) joint. Keep the hands near, but not touching the wrists unless needed.