Never knew that tapping the back of the phone did anything. Yet another Apple convention you only discover by accident?

It’s an “accessibility” feature. You can assign an action to double and triple tap. But it’s very inconsistent. It doesn’t work half the time and it will trigger randomly if you move your phone funny.

More like an accessibility setting that has widespread utility (there are several others, e.g. iOS has built-in background noise generators in accessibility)

That said, tapping on the back of the phone didn’t register consistently enough for me to utilize. I tried setting flashlight to that action. When I wanted it to work it wouldn’t. Then when I would be absentmindedly tapping it would activate.