Android 7.1+ has a ‘Panic Detection’ mode security feature
We are all aware how prone Android devices are to malicious app attacks. With this, Google embedded a security feature called “Panic Detection” to help fight back rogue apps.
Basically, the gist of this security feature is to count the number of times you press the back button. This is to differentiate a regular button press from the four button presses needed to activate this feature. If a rogue application tries to hijack your screen and prohibits you from leaving, Android itself will override the app to bring you back to your homescreen. Then from there, you can uninstall the malicious app from your launcher.
This feature didn’t make quite a buzz as Google didn’t want to advertise to the world one way they are fighting back malicious applications.
Source: XDA Developers