If you pinch a single plate, you must be sure that you pinch only the rim or the grip becomes useless. If your fingers are pressing against
anything but a smooth, flat metal surface you're doing it incorrectly. They can't touch the lip of the rim which runs around the plate; they can't touch any raised lettering; they can't touch any of that shit. If you've got that squared away, rotating your fingers to the point where you can employ your fingernails is counterproductive. It doesn't help, and it puts your fingers at a mechanical disadvantage. So your hands should look like this:
The thumb, which you can't see in that pic, extends also, almost as if you're making that little "blah, blah, blah" motion with your hand that some folks do. When you're pinch gripping like that, you'll certainly fail in your hands before you fail in your forearms.
Secondly, choose a weight that you can only keep airborne for about 5 seconds. Just like anything else, if you want strength keep the weight close to your max. You can choose weights that you can heft for longer than that, but you'll be training endurance more than anything else.
Lastly, plate curls do not waste time. John Brookefield says their one of the key exercises for good wrist strength and pinch grip. I would advise you to include them in your routine.
