iOS 14's Back Tap is an accessibility feature that literally lets you tap the back of your iPhone to access system features or start shortcut automations. For example you could assign your App Switcher to the double back tap, letting you open your App Switcher simply by tapping the back of your iPhone two or three times. There are a bunch of system and accessibility features available for Back Tap, including Control Center, Home, Screenshot, Magnifier and Zoom among others, but Flashlight is noticeably missing.
Back Tap makes it possible to perform a variety of system actions by tapping the back of the iPhone case. Actions can be assigned to a double-tap or triple-tap, meaning that up to two actions can be configured at a time. For example, double-tapping the back of your iPhone can scroll, capture a screenshot, change the volume, or even launch a Shortcut. Back Tap also provides options to launch various Accessibility features.