It does what it says on the tin. With Yorkshire Grit, it starts at about 300grit, but breaks down in use to about 1000grit so it can be used to finish "sand" from 240grit up, which is the finest and most dangerous dust nicely trapped in a mix of bees wax and mineral oil rather than floating around my workshop.
I don't bother to make it, a £10 tub lasts me between 6 and 12 months and puts a few coppers into Yandles stay open fund.