It can be used to add a fuse to the fuse panel in a slot that's already occupied, or an empty one. From what I understand, if there's a fuse in the slot you want to use, you take it out, plug in the add-a-circuit and plug the old and new fuses into the add-a-circuit. It has the red power wire running out to whatever new accessory you want to add. I'll be using it to hook up a few sets of cold cathodes and several LEDs, all running through a switch of course. I'm thinking I want it hooked up to constant power, but I don't know what slot in the fuse panel to use... anyone?
Oh, and I got it at Pep Boys for under $10, but I think Autozone sells them for around $5. PB is next door to my office, so the extra $ is worth not having to drive across town.
Here it is... Our xBs take the mini ones...