With Back Tap enabled on your iPhone, a quick double or triple tap on the back of your iPhone can be used to quickly trigger oft-used actions such as launching the Camera app, taking a screenshot, activating Siri, turning the flashlight on and off, trigger accessibility-specific actions, and more.
How to turn on Back Tap:
- Check that you have the latest version of iOS on your iPhone 8 or later.
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Go to Settings > Accessibility > Touch, and tap Back Tap.
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Tap Double Tap or Triple Tap and choose an action including, importantly, Shortcuts.
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Double or triple tap on the back of your iPhone to trigger the action you set.
MacDailyNews Note: You can also set a double or triple tap to trigger an Accessibility Shortcut to quickly access features like Siri Shortcuts, Magnifier, Reachability, AssistiveTouch, and VoiceOver.

This is very helpful, thanks, had no idea, though no camera option in iOS14. I set double to screenshot and triple to notification center, both awkward or inconsistent the traditional way.
Will this work with a case on?
Yes, it should work with a case on your iPhone.
Maybe not on the thickest cases but it works on my thin case.
Tap faster than you think you should