One of the strongest grip trainers I've met told me that, if he had to pick one move that made his grip stronger than anything else, it was picking up two heavy dumbbells, he mentioned something like 100-150lbs range, and walking with them as far as possible until you drop them. Then, turning around, and carrying them back. He said the key was to walk away from your house, then you will be forced to carry them back once you drop them the first time. Also said you may drop them many times on the way back, but to not cheat and keep going until you are back where you started. He was one of the earliest "captains of crush", and the strongest gripper I have personally met. He was casually shutting the Ironmind #3 while talking to me over and over, and was shutting it hard enough to where it was making a loud click. He could also do the same thing with the #3 held upside down.