Theatre Mode
Theatre Mode, which debuted back in 2016 as a means of making sure that your Apple Watch screen stayed off in a dark room even when you raised your wrist, is even more useful — and arguably necessary — now on the Apple Watch Series 5; in fact, other than disabling the always-on display feature entirely, Theatre Mode is the only way to turn the screen off.
In reality, it works the exact same way as on older Apple Watch models. It turns the display off entirely, and leaves it off unless you explicitly tap it to light it up immediately or turn the digital crown to gradually illuminate it.