The Nagura stone is used for raising a slurry on fine-grit finishing waterstones. This slurry is what actually does the honing and polishing of the sharpening stone, and the Nagura stone is wonderful for speeding up the process of slurry-making. The fine paste will polish the tool blade as it cuts it. Just wet the finishing stone and rub it with the Nagura stone. Rubbing evenly across the entire stone will help even out the wear on the stone so you won’t have to flatten as often. Nagura stones are sometimes called correcting stones
in Japan, because when they are rubbed on the high spots of a warped stone, they can help straighten out the surface. I typically use one on the ends of the 8000 grit stone to give me some initial slurry and also to balance out the wear on the stone so that I would need to flatten it less. 3/4" x 3/4" x 3".