This is basically an improvement on luminosity masks:
- You use a specific color channel (or any grayscale image obtained using the channel mixer)
- You use a Curve to define the area instead of operations which allows you to set them exactly where/how you want (the operations defined in PatDavid’s canonical tutorial can be done in Curves too, with straight lines at specific places)
Btw if you use Luminosity Masks in Gimp you can have a look at this