One of my favorite new features of watchOS 4 is using the Flashlight feature. It’s surprisingly bright for short distances, and sometimes a bit more handy than fumbling with my iPhone.
From the watch face, swipe up from the bottom and tap the flashlight icon. The screen will become all white, but somewhat dim. You can swipe from right to left to get a blinking white light to use as a signal, and swipe once more to get a red light that won’t disrupt your night vision as much.
As you turn your wrist away from your face, the screen brightens all the way to maximize the flashlight effect. Rotate back to your face, and after a moment of blinding yourself, it will dim again.
Apple’s support article says:
To turn off the flashlight, press the Digital Crown or side button, or swipe down from the top of the watch face.
I’ve found an even more satisfying way. You can also simply cover the screen with your palm, and the flashlight is extinguished. The best part is it feels natural enough to do without needing to rotate your wrist back toward your face, saving your eyes from that momentary bright light.