1- Set down on a bench with your dumbbell in each hand at shoulder level, underhand grip (thumbs pointing away, elbows bent and pointing forward.
2- Inhale and raise the weights up vertically while rotating your arms 180 degrees at the wrists, turning them into overhand grip.
3- Exhale at the end of movement then lower the dumbbells to the original position by rotating the palms of your hands towards you.
Tips and advises:
1- Don't swing the weights while lifting them up, keep a straight line with your arms.
2- Avoid too heavy weight especially if you're prone to shoulder or elbow injury, due to the nature of the movement.
Can be performed on a regular bench or while resting the back against a backrest to prevent excessive arcing in the back.