Opacity mask opacity?
Again, guys, a beginner's question - but here is something I don't get about masks in general, a difference in the way I would expect opacity to work and how it actually seems to work.
I'm using a still image as a sample:

Now I mask it:

And now I would expect that the upper of the two sliders I marked with arrows ("mask opacity") would allow me to fade the mask itself out so that the image below becomes visible again, while the other slider ("opacity") fades the layer itself out. Instead, both of them fade the remaining (unmasked) part of the image out:

What good is that? And is there a way to accomplish a fading mask without putting a duplicate layer below?

