Grasp a moderately weighted dumbbell so your palms are flat against the underside of the top plates and your thumbs are around the bar. Lie on your back across a flat bench so only your upper back and shoulders are in contact with the bench. Your feet should be set about shoulder-width apart and your head should hang slightly downward. With the dumbbell supported at arm's length directly about your chest, bend your arms about 15 degrees and keep them bent throughout the movement. Slowly lower the dumbbell backward and downward in a semicircle arc to as low a position as is comfortably possible. Raise it slowly back along the same arc to the starting point, and repeat for the required number of repetitions.

The diagram shows the most used muscles on this exercise