The dumbbell one arm bench fly is an exercise that targets the chest. To perform the exercise, you will need a bench and a pair of dumbbells. Start by lying on the bench with your feet flat on the floor and your knees bent. Hold a dumbbell in each hand and extend your arms out to the sides so that they are parallel to the floor. Keeping your elbows slightly bent, lift the weights up until they are just above your chest, then lower them back down. Repeat this movement for the desired number of repetitions.