This feature lets you turn off the screen quickly by double-tapping on the alphabet (or on the screen's left side).
To enable the quick-lock gesture, go to Niagara Launcher’s settings > Features > Quick-lock gesture.
How to enable the accessibility service?
Open Android's settings > Accessibility. Niagara Launcher's accessibility service is listed here or in a submenu (which could be named "installed services" or similar). Tap on this option (it should be named "Niagara Launcher") and turn it on.
I cannot enable the accessibility service
If you get the error message "Restricted setting" when you try to turn on Niagara Launcher's accessibility settings, follow this guide.
Enabling the service causes a button to appear. How to turn it off?
Why does the gesture only work on the alphabet?
If you could also double-tap on the home screen or app list to lock the screen, every app launch would take longer. That's because Niagara Launcher would have to wait a bit after you tap on an app to check if you're doing a single-tap or double-tap gesture.