Curves Luma does not always work?

Hey everyone.

Often, when I start a grade, I'll desaturate a node, and then go into curves to set my contrasts. I find that often, if I select the luma (Y) only, and start fiddling with that, nothing happens, no adjustments are made. If I link RGB again, then I can adjust. Same thing with the values under the color bars/wheels. Adjusting luma does nothing to the image.

I suspect that this is by design, and I was hoping that someone perhaps could help me understand what I am failing to understand.

Here is a reference of what I'm trying to do:

In advance, thanks!

It appears to me that the amount of luminance that results from the Y-Custom-curve depends on the L.Mix setting with zero producing none and 100 producing the maximum change. Whether that is linear or not I don't know. In any case I no longer have my Luminance Mixture default to zero.