The "Rebore-Zit" is a very specialized piece of equipment, designed for enlarging holes through which wires are drawn without losing the wire itself. (No big deal on a stud as shown, but a royal pain if it happens inside a wall. You pull the wire through the wall, attach it to an eyelet on the end of the bit that doesn't rotate, then jam the whole thing back into the wall until you've gotten the size you wanted. Good for precision work, but if it were just drywall, I'd probably just pull the wire through and rough out a sloppy hole.
I'm sure there are almost none of you guys out there that actually need this bit, but I find these kind of specialized tools fascinating.
Enlarge Holes Without Removing The Wire [Toolmonger]