STARTING POSITION:
Select the appropriate weight of the dumbbell. Tie the weight with a rope with a short bar at the end. Grab the bar in front of you with your arms parallel to the floor. You should use a prone grip.
MOVEMENT:
Moving your hands, fully roll the rope. Return to the starting position in a smooth movement unrolling the rope.
TIPS: While your are doing the exercise, move only the wrists, make sure you don't move any other part of your body.