Self-Centering Bit

A self-centering bit (also called a Vix bit or hinge bit, after its most common use) has a spring-loaded, conical guide sleeve surrounding the actual drilling point, which seats itself into the countersunk or beveled edge of an existing hole -- a hinge screw hole, a piece of hardware's mounting hole -- and mechanically centers the drill bit precisely in the middle before it starts cutting.

It solves a genuinely fussy problem: eyeballing the exact center of a small hinge screw hole by hand is unreliable enough that pilot holes routinely drift off-center, which either strips the screw's grip or visibly cants the hardware. A self-centering bit removes the guesswork entirely for exactly that repeated task.