Paul Moss wrote:

Stud Extractor.
It looks a bit like a square pyramid but very thin and tall. About
two or three inches long. During manufacture it is twisted into a
lefthand spiral.

The bolt must be drilled down the middle and then the extractor is
screwed into the hole until it jams in tightly. Once it jams in you
keep turning it and being a LH thread it should unscrew the bolt.
If it snaps then you will really have a problem as it is too hard to
drill with ordinary tools.

That's the one. My understanding is that they snap often enough to make them
a bad risk when you're faffing around with car stuff. Whether bike stuff has
less time to corrode or gets torqued up less, I've no idea.