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://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/