This is a variation of a standard barbell curl that works the biceps and forearms through a complete range of motion by rotating the wrist.
Stand upright with a dumbbell in each of your hands and your arms extended at your sides. Your palms should be facing upward. Begin the exercise by curling the weights until the dumbbell is at your shoulder height. Hold this contracted position and rotate your wrists so that your palms are facing the floor. Slowly lower the dumbbells back down to your sides. As you reach the bottom position begin to rotate your wrists so that they are once again facing upward. Repeat this movement until you have completed your set.
Make sure to keep your elbows close to your body and do not swing the weights up. Your upper arm should remain stationary.
When you begin to lower the weight with your palms facing down, your thumb should be higher than your pinky. This will make the movement feel more natural.