A stud extractor is a tool designed to remove broken studs or threaded rods that lack a hex head for a standard socket to grip. A common example is an exhaust manifold stud that has snapped off, leaving a short section of smooth metal sticking out of the cylinder head. The extractor fits onto a ratchet or impact wrench and uses a cam roller mechanism to grip the stud.
The interior of the tool features three knurled steel rollers. As you turn the tool counterclockwise, the rollers slide down curved ramp walls, clamping tighter and tighter onto the smooth metal shaft. The harder you turn, the tighter the tool grips, allowing you to back the stuck stud out of the engine block. Because the rollers bite into the metal, they will ruin the threads on the exposed portion of the stud, so the stud must be replaced.