This exercise is sometimes called the Butterfly Swim and is a great dumbbell shoulder exercise.
Kneel down by the side of a suitable sized Swiss Ball, with your chest in contact with the central part of the ball, enabling you to have a good range of movement with both arms, whilst holding the dumbbells.
When doing Butterfly Swim, avoid using a ball that is too large, as this will restrict your shoulder movement and cause the dumbbells to his the ball.
For best results, hold a small dumbbell in each hand, and smoothly rotate both arms around in a complete circle, aiming to keep the arms straight and close to your body line throughout the movement.
Focus on keeping the movements smooth and under control, taking note that as the weight extends further away from your bodies central line, the heavier it will become. As dumbbell shoulder exercises go, this is a good one.
Whilst in the same position, you may also decide to perform a breaststroke swimming action, aiming to keep the weights horizontal throughout.
Aim to keep yourself positioned high on the Swiss ball in order to have a full range of movement.
If you feel any pain, especially in the lower back, stop this exercise immediately.