Stud Extractor
From DT Online
Stud Extractors are tapered left-hand threads of different sizes made in hardened steel.
If a Stud, Bolt or Machine Screw is sheared off inside a threaded hole, it can sometimes be removed by drilling a pilot hole down its centre then screwing in an Extractor.
Because they are left-handed, the Extractor will start to unscrew the broken Stud as it tightens inside the pilot hole.