If you have an iPhone 6s or 6s Plus, 3D Touch can be used to quickly change apps. The app switcher view is usually opened by double-tapping the Home Button, but now the pressure-sensitive screen gives you an 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.
Guide: Using 3D Touch to switch apps on iPhone 6s | iOS 9
- by Tom Rolfe