To perform the arc row, set up as you would for a regular dumbbell row with hips back and a flat back; upper body close to parallel with the ground. Begin the exercise with a pronated grip (Palm towards your body). Move your arm out in an arc (hence the name arc row) and gradually supinate (palm facing away from body) your hand as you bring it back and touch your back pocket. Be sure to pinch your shoulder blade as you move towards the back. If you do not do this, you will just get shoulder extension which is not what we want. We want scapular retraction and external rotation. Once you touch your pocket with a supinated grip, rotate back around to the starting position with the same arc you used during the concentric portion of the exercise. These are best performed for 2-3 sets of 8 to 12 reps at the end of a workout or as part of an extra workout.
Give them a shot next time you train upper body!