What is the use of the hidden anchor in the MUSHRA method?

Is the anchor hidden? I thought only ABC/HR had a hidden anchor.

I can see a use for a hidden anchor or anchors if you're worried about spoofing or deceptive subjects.

There could be other reasons, actually. In general, I support the idea of test controls, you never know what you'll find out.

The main idea of the anchor as far as I understand is to provide a sort of second "fixed-point" for the scale and thus make the results easier to average from one listener to another. Without that, it's hard for the listener to decide how to rate the artifacts because there's only one absolute (the reference). Usually, the anchor is going to be rated rather low (e.g. around 20 on a 100 scale) so everything else will be between that and the reference. If someone then rates a file 60/100, then it means they think it's half-way between the reference and the anchor.