Jed Hartman

On an iPhone 8 or later, you can set the phone to perform various actions when you double- or triple-tap on the back of the phone.

(Article from 2020, but it still works.)

https://www.howtogeek.com/690823/how-to-take-a-screenshot-by-tapping-the-back-of-your-iphone/

How to Take a Screenshot by Tapping the Back of Your iPhone

If you own an iPhone 8 or later running iOS 14 or later, you can now take a screenshot just by tapping its back thanks to a new accessibility feature called Back Tap, which you’ll have to configure for the task. Here’s how to set it up.

How-To Geek
May 10, 2023 at 5:27pmWeb
Show threadEric StephanMay 10, 2023
@elysdir true but when I have configured those, the thing fails to trigger half of the time. My daughter proudly tries to use that feature and we wait a while for a successful flashlight to appear. Tap tap... wait ... tap tap
Show threadLeaping WomanMay 10, 2023
@elysdir cool, I hadn't known that! Am giving it a try and it works well so far.