This is a neat little shortcut that you can programme yourself. Head to Settings > Accessibility > Touch > Back Tap and then you will see a list that says Double Tap and Triple Tap. Select one of those and it will bring up a list of functions you can activate by tapping twice (or thrice) on the back of your phone. Useful ones are the camera and torch, but you can also use it for Screenshots, Siri, Volume up or down, plus lots more.