It's a pretty complex mechanism here, but basically when a key is depressed it lowers a string onto to the wheel to sound. One string per note, unlike the hurdy-gurdy. There are other examples of this idea which goes back to the geigenwerk or Viola Organista, supposedly invented by Da Vinci. Here's a one being played with aplomb!

https://www.youtube.com/watch?v=sv3py3Ap8_Y

I especially like the Marais!

The wheel harp is also work a look:

https://www.youtube.com/watch?v=zUk8rLGxb48