On Tue, 01 Nov 2016 20:08:17 +0000 (GMT)
Stuart wrote:

Electrical discharge machining.

yes

Also know as spark erosion. A well known technique for removing broken
taps from metal.

so that is how they do that

Depending on the capability required, they can be easy home-shop
projects.

are there off the shelf units
i had never heard of it but now will have to look some time

in this competetion the operator could not see as it progressed
but instead listend and watched the bubbles

seems to work in a variety of material too
steel aluminum and pencil lead

wonder if it could be used in wood