However you run the shortcut, you now have a number of options to choose to not only lower the brightness of the screen, but also increase it. At the bottom you'll see the "Max" option, which is simply 100% brightness. As you go up the list, the brightness will decrease: 85% for "Higher," 71% for "High," 50% for "Medium," 36% for "Average," 10% for "Low," on and 0% for "Very Low."
The first setting on the list, which is "Minimum" is the one you're interested in. Aside from dropping brightness to 0%, it also turns on a couple of accessibility settings to help dim the screen. The first is Reduce White Point, which reduces the intensity of bright colors, while the other is Zoom, which zooms into the screen and dims it (if toggled on), for those with visibility issues.
If you tap on "Minimum," you'll first need to give the shortcut access to your Settings, but then the brightness will drop significantly, to the point where you may not be able to see the screen if you are in the light. In fact, you may have to go into a dark room to see what's on the screen, which is why you should only use this option at night or in a dimly lit location.
You should also be aware that the first few times you use this shortcut, it'll zoom in automatically into the screen, which is something you probably don't want. All you need to do is use three fingers to double tap on the screen and you'll be zoomed back out to normal, but maintain the extremely low brightness. After a few times of using the "Minimum" option, you won't be zoomed in because your iPhone will recognize that you don't want to be zoomed in.
Although you can increase the brightness manually using the brightness slider, this won't disable the two accessibility features that are turned on when you use the "Minimum" option. Instead, run the shortcut again and choose any other brightness option, which turns both features back off and increases the brightness.