If you have a 3D Touch-enabled device (iPhone 6s and newer) it’s possible to use the display’s pressure sensing capabilities to quickly change apps. The app switcher view is usually opened by double-tapping the Home Button, but this is an even quicker alternative option. You can also launch the app switcher by swiping right from the left-hand edge of the display while pressing hard on the screen.

Quickly change apps: launch the app switcher with 3D Touch | iOS 11 Guide
- by Tom Rolfe