Looking at a picture should give you a good idea how they work and are used. It's just a rig for offsetting the spindle holding the cutter (by up to 6"), and rotating it about the machine's quill. This combination of offset and rotation means you can do all sorts of radius cuts, located by the table's x/y travel, then take off tangentially at any angle using the Volstro head's 6" travel to another tangent (or otherwise) location, move the new radius center under the rotational axis (the main machine quill/spindle), set the new radius on the Volstro head, and swing the new arc, repeat as needed. All without breaking the setup. As long as you don't need a straight longer than 6", you don't need a rotary table involved. If you do need a longer straight run, you can do the setup on a rotab, make the long travel using the mill table, and the short travels/arcs with the Volstro (assuming they don’t exceed it’s capacity). Without the Volstro, just using a rotab, you would need to break/make your setup repeatedly as you move every arc/radius center axis to the rotab axis.
It's so amazingly flexible you can, with a little thought, cut just about any 2D pattern you need.
As others said, this is just the more commonly recognized Volstro product. There are also 90* horizontal heads, shapers, and high speed multi-axis quills. And there are other options than the Volstro rotary head for that functionality. The best is the one (IIRC, “R” head?) made by Bridgeport (also called a Cherrying Head). And the Bridgeport slotting head is also much better than the Volstro version which is comparatively light.
