To set up your own Back Tap shortcuts, Kaya explained you can go to your iPhone’s settings, then Accessibility, then Touch and then Back Tap. In addition to opening the Camera and Flashlight apps, you can choose from a number of actions including mute, screenshot, volume up or down, various accessibility shortcuts and more.
Article source: https://www.huffingtonpost.co.uk/entry/7-extremely-cool-things-you-probably-didnt-know-your-iphone-can-do_uk_63f4b6b1e4b0a1ee1495e941