Update:
While true that the forearms receive indirect work from gripping heavy weights, especially on pulling movements, direct work will always grow muscles best. Wrist curls, reverse wrist curls, hammer curls, and reverse curls all have their place in bodybuilding. Nonetheless, overly large or small forearms both harm the aesthetics and balance of a physique, so forearm exercises must be considered individually.
Most try to get bigger forearms with hammer curls, wrist curls, and reverse wrist curls. They may fight the burn using a wrist roller or gripper. They try to vary things by performing the same exercises on a preacher bench, behind-the-back, with an EZ curl bar, and so on. If they consider themselves educated, they may do supination, pronation, radial and ulnar deviation exercises. They may add thick bar lifting. Many of these cross into the territory of working the hands which contributes to your grip.
The forearms give a subtle display of power. You can show powerful forearms while wearing most clothes. They also enhance your grip, such as when catching fish with your bare hands or squeezing an orange for no real reason. It would feel good to possess the sort of dominance that allows you to grip a chump’s hand, knowing that with a slightly off remark, you could mush his hand into dough for cookies. At the least, the chump would seem afraid after seeing the orange.
If you have followed the other bodypart articles, then you should know what to expect by now. You need none of these exercises and will find they harm the wrists. The forearm and hand muscles exist to grip things, and just by pulling heavy weights through dead-lifts, pull-ups, or rows you should work these muscles plenty. This also includes walking with heavy weight, rope climbing, and odd object exercises.
Facts
If you perform wrist curls, you place tremendous stress on your wrists. Imagine trying to snap a pencil with your hands. This mimics the effect on your wrist during these movements. The forearm and hand evolved to work as a unit. Many small muscles work together to maintain a grip. Wrist movements undoubtedly train the forearm muscles, but also pose a great risk and generate less tension for growth, as all isolation exercises do.
The wrist will slightly extend when closing the hand to keep favorable muscle lengths for each muscle in the forearm. The muscles can perform flexion, extension, abduction, adduction, pronation, and supination. Although movement is possible in all directions, these actions occur unloaded in real life. The hand alternates between mobility and stability. It remains most stable when grasping an object.
You can test this for yourself. Make a fist as tightly as possible. Notice how all of the muscles throughout the forearm tense up. The only muscle that may fail to receive as much stimulation, the brachioradialis, gets worked when you flex the elbow during a pull. Curls could never create that sort of tension in that many muscles. You will feel no strain in your wrist during this as well.
A Pull is Enough for Bigger Forearms
In the rare case that you deem extra work necessary, focus on gripping alone. Hold a barbell or two dumbbells in front of you or at your sides for time. End the set when forced to let go. You could up the weight by 5 pounds each time you can hold it beyond 30 seconds, or perhaps a minute. You could use a power rack with the safety bars placed higher when using a barbell, to prevent too great a fall when you drop it. Consider some of the options mentioned here for grip training. The video there shows you how to do the barbell exercise just described.
Make sure you wrap your thumb around the bar to apply the power grip for any exercise. A false grip, in which the thumb aligns alongside the fingers, fails to work the muscles as well and makes the grip less secure.
Focus on the basics to build bigger forearms. Avoid movement at the wrist. Just because you can move the wrist does not mean that you should move the wrist under a load. Add weight to your pulls, and consider a grip exercise only if needed.