Grasp unilaterally loaded dumbbell with thumb next to weighted side. Lie on bench on side with weight and position arm with "half dumbbell" on bench. Bend elbow approximately 90-degrees and tucked bent elbow under body. Position thumb down (supinated).
Rotate dumbbell so thumb turns upward (pronation). Return and repeat.
Bench can be flat or slightly incline (0-30 degrees) so resistance occurs during initial range of motion. If unilaterally loaded dumbbell, or "half dumbbell" is not available, grasp conventional dumbbell to one side of handle with pinkie against inside surface. Also see conventiontal Dumbbell Lying Pronation.