Stand with your feet shoulder-width apart and hold the dumbbell down and to your front. You should be gripping the dumbbell with your palms facing toward you (reverse grip). Your other arm can be on your side or braced up against your stomach. This is your starting position.
Raise/curl the dumbbell up until your forearm is almost completely vertical. It's important not to move your upper arm too much - move it as little as possible. Once at the top, curl the dumbbell back down to the original starting position. Finally, repeat this entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't swing the dumbbell up in an attempt to lift it; this is entirely counter-productive
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use a heavier dumbbell.
This is an advanced exercise for the forearms. It requires a strong grip, strong back and strong mid-section. If you're a beginner and wish to do this exercise, you may want to reconsider and select a simpler forearm exercise - one that doesn't involve the use of dumbbells (see complete Forearm Exercises list).
This exercise will build your forearm muscles really quick. However, you have to get the movement right. If you move your upper arm too much then you will be targeting your upper back (lats). Thus, before doing the exercise, you may want to do a few practice repetitions with a light dumbbell just to get a hang of the mechanics of this exercise.