While Back Tap is meant to be an accessibility function, the new iOS 14 feature for iPhone is an extremely convenient way to launch apps, run Shortcuts, or any number of things.
Apple built Back Tap to create a simple and easy way to trigger convenient tasks for those with accessibility needs. A quick double or triple tap on the back of your iPhone can be configured to trigger a number of actions, such as opening the Magnifier, or starting VoiceOver when your iPhone is in your pocket. It can even map to common system features such as Control Center, or accessibility-specific activities such as Switch Control actions.

William Gallagher for AppleInsider:
You can open Control Center, lock your iPhone, turn on the torch, or start any of around 20 other system functions. Once you’ve got used to this, your one regret will be that you can’t have a tap do more than one thing. But, you can.
Tap Back is limited to either a double-tap, or a triple one. However, you can set either or both options to run a specific Shortcut, and then that can let you control as many functions as you want.
You do need an iPhone that can support the feature. However, all phones from the iPhone 8 onwards do.
MacDailyNews Note: Back Tap is easy to enable and use on your iPhone – click thru to the easy instructions here.
Its slow on my 11 Prol
Things that i need but shortcut app can’t do :
1 – clear safari history in 1 tap and scheduled clearing of history
2 – Turn on and off dapp delete through screen time
Back tap is worthless to those of us who use a sturdy case. And that is almost a requirement nowadays. I’d rather go without that functionality than risk a huge repair bill if I drop the phone without a case. Not to discount it but I’m sure many agree.