Knock twice as if you knock a door near your phone or over screen to wake up your device. Phone must be on a flat surface transmitting vibrations like table. To be able to detect knock knock, firstly you must activate application by pressing power image in main interface. Notification appears when the app is activated. You can open or close it any time in main ui. The app runs in background after it's activated and begins listening vibrations around your phone.

Warning: Battery drain may occur at long usage of the app since the app listens for knock knock vibrations all time.