This accessory exercise is great for powerlifters and anyone who lifts heavy and needs more grip strength.
It’s really a very simple movement. Get a big bucket and fill it with rice. I’ve also seen rubber chips, metal shot, and sand used. The metal shot is hardcore and should only be used by those with a very strong grip to begin with. Rice is your best bet as it’s the easiest to use and you can always progress up to the sand from there.
I made mine with a big laundry tub I found at the dollar store and a huge bag of rice from a warehouse club. Once you get it set up all you do is dig your hands in and begin squeezing. Pretty simple stuff, nothing too complicated.
I’ve also found I get great results when I twist my wrists both clockwise and counterclockwise as I squeeze. This is best done to failure at the end of your training session.