Back Tap is an accessibility-related functionality, and Apple has designed it in a way for anyone to take advantage of. Once enabled, Back Tap allows you to bind various iPhone features such as Siri, the Control Center, and even shortcuts. As its name implies, you can then enable it by tapping the back of the device. Back Tap also relies on the built-in accelerometer, so you can set it up on any iPhone that's capable of running iOS 14.
You have the option of binding a double and a triple tap to various iPhone features. I am using screenshot for one of those but you can even bind to Shortcuts scripts as well.
See how at How to Enable and Use Back Tap on iPhone
Want to enable and use the iOS 14-related Back Tap functionality on your iPhone? Here's a guide on enabling Back Tap on your iPhone - well, it works with iPhone 8 and higher.