What's inside a cheap insulation tester?

I bought a Yu Fong YF-509 years ago, and now I've got two. Let's see what makes them tick, and more importantly how they generate the test current.

Schematic in PDF format (download it for a proper high-res view) at https://drive.google.com/file/d/1tBiQ...

INDEX
0:00 Index
0:10 Introduction: Yu Fong YF-509 insulation testers
0:55 What can they do?
1:16 Checking the output voltage
2:46 What's so special about the big, fat probe?
3:05 A look around the outside
4:58 What's inside?
6:48 PCB close-ups
7:20 Schematic and (some) theory of operation
16:18 How much current does the insulation test put out?
17:06 Can you feel that?