A paint brush spinner is used to clean paint brushes or roller covers. With water-based paints or varnish you simply insert the brush handle into the tongs or grippers, or slide the roller cover over the outside of the tongs. Then while running the brush or roller cover under water from the utility sink (or garden hose and garbage can) you get it spinning.
The brush spinner has a spiral shaft that when pushed into the body rapidly spins the brush or roller cover. The whole process is similar to a child’s spinning toy, with the end result being that dirty water and paint is flung off. A similar process can be used with oil-based products.
The only downside is that the spinner spiral shaft eventually gets rusty and needs lubrication. In the past I’ve used DL Waterless Hand Cleaner to remove any rust that builds up.