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