This is one of the best solutions for a very common problem that I’ve ever found in Photoshop. To understand why it’s effective you must understand first that you can control the opacity of your brush tool.
You have the freedom to choose which ever opacity you'd like by using the opacity slider at the top Photoshop where each set of tool options are. The less the opacity, the more subtle the effect of brushing is.
OR as a shortcut you can press any number from 1 - 10 on your keyboard while having the brush tool enabled and it will give you the corresponding opacity
Now that we all understand brush opacity, this FADE method might have a bit more value to you as a power user.
The FADE tool is a shortcut to choose your opacity after a brushstroke! That's right... AFTER!
One important thing to note is that it only works on the very last action/stroke that you've done with the brush. So this means, it will work best if you do as much as possible with one brush stroke before moving onto the next one. And use the fade brush tool in between each larger brush stroke.
Say you start with one 100% opacity brushstroke. All you have to do is make sure you do all of your painting with one stroke. Once you've stopped that brush stoke, you then toggle the fade brush tool to choose your opacity after the action.
It becomes immensely valuable when doing these very subtle effects and gives you something I think is important which is the visual option. You get to see the opacity change on the brush stoke and then stop it when it gets to that perfect visual place.
You can either find this tool in the edit menu...
Or you could memorize this beautiful shortcut
Command + shift + F
The important designation between the fade brush tool and choosing opacity before you start brushing is that even if you pick a 10% brush you will only ever see increments of 10 while you are painting. This means you only ever get to see what 10% looks like, 20%, 30%, 40% and so on.
You will never get to visually see what something like 23% or 47% looks like because it won’t be an option.
It's important to remember that this not actually called the fade BRUSH tool but instead its just the FADE tool. It works with almost all brush tools as well as the fill tool as well as a few other things that I'm not thinking of.
I hope you love it as much as I do.