There are different ways to turn on flashlight on LG V10 from using the shortcut in the notification panel to installing 3rd party apps. But the quickest way to turn on flashlight on LG V10 when the screen is off, is by doing as follows:
1. Open the Settings app (Switch the view to List by tapping the 3dot icon on the top right followed by the list view option)
2. Under the Device settings, tap the Second Screen option
3. Make sure the "Show when main screen off" option is enabled
4. Now turn of the screen and you should see your signature or Date & Time in the second screen
5. Simply swipe the second screen to the left to unveil the 4 quick buttons.
6. Tap the Flashlight to turn it on when the screen is off