the cable under test passes through a bunch of steel beads that is connected to the high voltage output and the cable core is connected to earth.
The place I used to work at had those on the machines that made sparkplug cable. They onl detected a bad spot in the cable, not a measurement in the amount of "bad". They only detect when the high voltage goes to ground, the high voltage was a set amount.
Going back MANY years we used to repair equipment for a local company, they were called 'holiday detectors' (and according to google still are), essentially they used a small inverter to generate about 500-600V (going from memory), and this was used to detect pin holes in painted surfaces. The company did research on rusting metal underwater, such as on oil rigs.
Presumably it was asimilar type of unit?.
I said MANY years ago, as I recall the units used Germanium transistors