Google Pixel phones have a secret button. Here’s How to Find (and Use) It

To access Quick Tap, open Settings and scroll down to System > Gestures. Then, go to Quick Tap to initiate the action. The page will display an animation of how to use the feature and a list of what you can activate, including taking a screenshot, opening the Google Assistant, playing or pausing media, showing notifications, and opening the desired app.

Too: How to automatically decline robocalls with your Pixel phone

Personally, I have Quick Tap set to turn my flashlight on or off.