Position
Sit or kneel in a ready position facing a bench. The bench should be set in order to align the trunk of your body between 45 and 90 degrees to the floor. Hold the dumbbells perpendicular to your body. Keep your arms straight down and your elbows pointed out and slightly bent. Pull and hold your shoulder blades tightly together throughout the exercise. Begin with your arms slightly out so that there is some degree of gravitational pull on the dumbbells, providing resistance.

Technique
Contract your posterior deltoids. Begin bending your elbows and pulling your arms apart and up. Pull your arms up and your elbows out until your upper arms are approximately parallel to the floor and form about a 90-degree angle. Hold, continue to contract your posterior deltoids. Keep your arms relaxed, your elbows pointing up, and your scapulae retracted. Slowly lower your arms back to their original starting position, keeping your scapulae retracted and tension on your posterior deltoids. Repeat.