here are how i train my forearms
I used to own a gripper similar to the one in the video. It was reconditioned with the bottom handle slightly misaligned. It also did not have the extra "leg" at the base, nor the rotating collars on the handles - hence, it was a bit wobbly and would stretch and burn the skin on the fingers during each rep.
Still, it was extremely effective!
I also fashioned a wrist roller out of 3-inch pipe with a longer cord. Standing on a bench with the weight starting on the ground, I can crank out a few more reps in each direction.