The way I see it, the act of gripping a heavy object and balancing it is heavily involving the forearm extensors and flexors. Watch the muscles in your forearm contract from relaxed to making a fist. If that's not a contraction then I need to get my eyes checked. Now bring some heavy weight on a barbell into the equation on deads and there's no question the forearms are being heavily worked. Do this on a regular basis with heavier and heavier weight, and they'll grow.
I think static holds (deadlift holds, farmers walks, etc.) are a great way to improve your grip but the act of deadlifting (essentially gripping and balancing that heavy barbell) with more and more weight over time is probably going to build them up more than anything else. I really can't see how doing wrist curls, hammer curls, or reverse curls with a fraction of that weight is going to stimulate more growth than that.
I've said it before, but I can't remember the last time I saw an experienced powerlifter with skinny forearms. Of course genetics plays a big role, but there's no reason why even a person with small forearms couldn't build them up to a respectable size from heavy deadlifting and regular grip work. Unlike gains with the biceps, you'll see visible progress over years rather than weeks or months.
Let's say after I did my regular rack pulls for 250lb.
I do static holds at the top of the movement for 350lb.
Will this help my forearm grow bigger? Or does it merely improve my grip?
Actually, I took some photos yesterday after reading this thread (but forgot to post them here) just to highlight that heavy deadlifting and static grip work can build up the forearms, or at least it worked in my case. It's probably difficult to demonstrate it without showing before photos but at 6'1", 150lbs, I can assure you there wasn't much there.
I did do hammer & reverse curls for quite some time before switching to weighted chins with those grips, and I'd say it all helped to some degree but there's nothing quite like heavy deadlifting. All that aside, the benefits of having a strong grip far outweigh having big forearms but a weak grip so I feel grip training should be a priority because it gives you more control over the heavy weights you're handling.
Haha, you're right, taking photos with a flash in front of the bathroom mirror isn't that flattering but it was the best I could come up with
I just wish I had some 'before' photos to illustrate it better. Some people seem to think unless you start training with big forearms they'll never reach a decent size but I never bought that line. It takes time but they do grow. I'm not against using straps even though I don't own a pair personally however I think the best way to build them is to do plenty of heavy deadlifting without straps (saving them for the heaviest sets) and lots of grip work (farmers walks, deadlift holds, etc.).