The Blend-if feature is easy to miss if you're not looking for it in Photoshop, but it's one of the most useful tools you can have in your post-processing kit. This great video will show you what Blend-if is and how it works as well as some useful examples.
Coming to you from Nemanja Sekulic, this helpful video will introduce you to one of Photoshop's lesser-known but very useful features, Blend-if. The idea behind it is simple: by adjusting the sliders, you can tell Photoshop how to have two layers interact with each other based on the luminosity values each contains (or their individual channel information). By then feathering this adjustment, you can very quickly do things like blending effects solely into the highlights or shadows or revealing parts of an underlying layer to make the two constituents look as one. The beauty of it is that because it's a global adjustment, it allows you to work very efficiently and effectively without having to mask anything in; nonetheless, because it's attached to a specific layer, should you wish to be more precise, you can easily add a mask and start shaping things as you please. It's by far one of my favorite features in Photoshop.