This exercise is used to build the rear deltoids. The rear deltoids are important when doing bench press and other compound exercises.
Hold a dumbbell in each hand and bend over at the waist until your torso is almost parallel to the floor. Your arms should be hanging down in front of you, and this is where you start the exercise. Now, lift the dumbbells out to the side until they are parallel with the floor and even with your torso. It is important to keep a slight bend in your elbows during this exercise; it will keep the stress off of your biceps tendon. Contract your rear deltoids at the top of the movement, and hold this position for a second. Then, slowly lower the dumbbells back to the starting position. Repeat this movement until you complete all of the repetitions in your set.
Keep your torso still during this exercise, avoid swinging the weights up. Control is the key to isolating your rear deltoids.
This exercise can also be done on a cable machine, or it can be done with a resistance band by pulling the band apart.