What is the difference between a "Luftresonanz" zither and other kinds, technically?

The luftresonanz is a second sound hole that is above the block where the tuning pins are. What it really does is give the back a larger resonating surface which enhances the bass. Most modern harfenzithers have it. The instruments that don't have it are usually smaller and often not as rich sounding. This is a fairly gross generalization but having a larger vibrating plate for the back usually means more bass response as well as increased volume. Hope this helps.

Ken Bloom
http://www.boweddulcimer.org

It certainly does. Thank you! Is this second space linked to the main open space, or is it completely separate? So does the block into which the wrestpins are screwed "float" in some way to avoid touching the sound board?

It does float. There is a narrow opening between the block and the sound board. If you blow air into the sound hole everything will come out the luftresonanz hole.

- Andy

That is why it works so well. It frees up that upper part of the back so it can move. A larger vibrating plate translates into more volume and bass response.

Ken Bloom
http://www.boweddulcimer.org