More like a brass agitator
if you have ever seen one, a brass tumbler is one of the reloader's best friend
. Next to the reloading press
It consists of a base which has a small motor inside and a covered dishpan that holds your spent cases and polishing media. The idea is to dump all your brass in it and, if desired, to mix some polishing solution and to tumble or vibrate the hell out of the spent cases for half an hour or so.
The rubbing of the polishing media, which can be anything from cracked walnut shells to dried and ground corn cobs, with the brass cleans it and gets rid of tarnish or other fouling. Makes it look like pretty and new again.
Most reloading companies sell a tumbler of some sort, the better known ones are Dillon, Lee, Lyman, and Hornady.