How did the QR code appear on the back cover of Apple X after knocking twice?

How to set the Apple phone to tap twice to open the health code? Here's the method.

Device: iPhone 1 1

System: iOS 14.3

1. First, open the shortcut command in the iPhone.

2. Then click the+sign to create a new shortcut instruction.

3. Then in the page that opens, select Open Health Code.

4. Turn on the auxiliary function in the phone settings.

5. Then click Touch and click the back.

6. Finally, tap twice or three times to select the health code of the shortcut instruction we just created, as shown in the figure below, and you can set it successfully.