The picture above instantly explains this trick. The pulse measurement LEDs on the back of an Apple Watch made an extremely bright flashlight should the need for one arise. Just remove the Watch, unlock it if you need to via your PIN, and then swipe up to the Heart Rate glance. Following this you’ll need to hold the Watch by its strap in order to avoid touching the screen or the LED sensors, which could turn the LEDs off.
The illumination lasts for around a minute.
Of course, doing this will mess-up your daily Heart Rate stats, so this isn’t a perfect trick. The light is also green, of course, rather than white.
But if you need illumination in the dark and your Apple Watch is the only thing at hand (literally) then it’s worth remembering.