I see on some violins there is something on the A and E strings between the tail peace and the bridge.

What is it called and what does it do?

[![Mystery object](https://i.stack.imgur.com/Gnxey.jpg)](https://i.stack.imgur.com/Gnxey.jpg)

I found a similar thing on the violins of the soloist in these videos.
I don't know if these are different from the picture above:

(look at 4:20):

This one is only on the D string:

Here everybody has it on their instrument, violin, viola, cello (look at 7:59):