This fascinating tool, made by Detroit-based precision tool manufacturer Cogsdill, is called a DBFM (Diamond Burnish Face Mill tool).

It mounts into a standard milling arbor, and each of the posts have a spring-loaded diamond insert in the tip. When the tool comes into contact with the metal workpiece, the springs force the diamonds against it, and it then produces a mirror-like finish.

As an example, the top half of the workpiece below shows a merely milled surface, whereas the bottom half has been finished with the DBFM.

Titans of CNC Machining got their hands on a DBFM, and used it to finish a disco ball that they milled out of a chunk of aluminum: