Grasp dumbbell in one hand. Lie on 30° to 45° incline bench with opposite side of body on incline, arm over top of bench, lower leg positioned on front side of seat, and upper leg on back side of seat. Position dumbbell inside of lower leg, just in front of upper leg.
Raise dumbbell from until upper arm is perpendicular to torso. Maintain slight fixed bend in elbow throughout exercise. Lower dumbbell to front of upper leg and repeat.
In order for lateral deltoid to be exercised, dumbbell must be raised by shoulder abduction, not external rotation. Also see exercise Dumbbell Incline Lateral Raise performed on preacher bench.