I bought one myself just for the fun of it, I mean £1.77 including delivery, who wouldn't?
Just sticking a single 18650 on its inputs unleashes an insane venom that makes you jump out of your skin on its output, the audible crack is not captured on any video. I can understand why Big Clive had to have the calming influence of alcohol before shoving his finger tip into the spark.
Now there are youtube vids on how to make a stun gun out of it with a battery, push button, and a couple of nails. Also an "EMP" device using a coil.
Pretty boring. The stun gun thing will get you a mandatory minimum 5 years in prison in the UK - no exceptions whether used in anger or not.
So it's a mildly interesting device. I've burned through paper and fruit with it, but it's not useful as an EHT source because it needs that bloody spark gap to stop killing itself!
400kV is also ludicrous. It's in the 10's of kV. Though I do believe it's more than 30kV as that was the old LOPT CRT voltage on old TV's and they never gave out anything like as nasty as this thing!
I wonder if as well as ozone it's belting out X-rays as well?