Kneel down and place your forearms on a flat bench so that your wrists are past the edge of the bench. Grab the dumbbells with your palms facing down (reverse grip). This is your starting position.
Curl the dumbbells down with your wrist as much as you comfortably can. Then, curl them back up as far as the natural range of motion of your wrists allows. Finally, repeat the entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't allow the dumbbells to jerk your wrists down as it may result in injury; always be in full control of the weights
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.
This exercise is not for beginners. It requires some previous experience with forearm workouts because dumbbells require a lifter to balance them and have a strong grip. Thus, beginners are better off doing simpler forearm exercises that don't involve the use of dumbbells.
Another point to mention about this exercise is the fact that no matter how advanced a weightlifter is, this exercise will surely leave any person's forearms sore for a few days after doing it. But, that is normal and only means that you have "hit" the forearms muscles well.