If you are planning a trip to Switzerland, I found on wayback machine of their old website they used to have contact details for those who wanted to arrange a visit:
They have since gone quiet, perhaps in part due to a scandal involving one of the inventors of the device, which may or may not be true. When he died an obituary article about him contained a short interview with a representative of the community saying that they wished to be forgotten:
I will have a look at it in September, personally.
If you are planning a trip to Switzerland, I found on wayback machine of their old website they used to have contact details for those who wanted to arrange a visit:
https://web.archive.org/web/20130501143953/http://www.methernitha.com/en.html
They may still work, although the given address is now a Shelving Manufacturer:
https://www.google.com/maps/place/Moosb%C3%BChlweg+2,+3673+Linden,+Switzerland/@46.8485731,7.6786066,17z/data=!3m1!4b1!4m5!3m4!1s0x478fb5af4e9a38a1:0x693dbaaef7f042f!8m2!3d46.8485731!4d7.6808006
I don't know if that's their business, or they sold some land
I am Swiss, but have not been there since 12 months.
https://en.wikipedia.org/wiki/Methernitha
Their device resembles a wimshurst machine, which collects electrostatic electricy but must be cranked (by hand).
https://en.wikipedia.org/wiki/Wimshurst_machine
How a Wimshurst Machine Works (video)
Methernitha's Testatica appears to have a very similar set up, but more elaborate and advanced looking:
https://rimstar.org/sdenergy/testa/misc/3kw_unfinished_garden.jpg
They claim that it is hand cranked originally to get it going, and then can run freely under precise circumstances:
https://rimstar.org/sdenergy/testa/methernitha_testatika_technical_info.htm
They once invited 30 retired engineers to a demonstration:
https://rimstar.org/sdenergy/testa/report99.htm
They have since gone quiet, perhaps in part due to a scandal involving one of the inventors of the device, which may or may not be true. When he died an obituary article about him contained a short interview with a representative of the community saying that they wished to be forgotten:
https://www.bern-ost.ch/Linden---Vatti-Baumann-gestorben-22016
And indeed their website is just a blank page:
http://www.methernitha.com/
And here is a decent article (in German) on the whole thing:
https://gehtanders.de/wolfgang-wiedergut-testatika-und-raumenergie/
And an accompanying video:
https://youtu.be/S4fgRpa2zcw
Also in German, unfortunately; However youtube's auto-translated subtitles to English do a tolerably "good" job, with German specifically.