You can press the Volume buttons to turn an accessibility feature of your choice (TalkBack, Switch Access) on or off.

To enable the shortcut:

1. Go to Settings > Accessibility.
2. Touch Volume key shortcut.
3. Turn it on .
4. To change the accessibility feature associated with the shortcut, touch Shortcut service.
5. To use the Volume buttons from the lock screen, turn Allow from lock screen on .

To use the shortcut:

Press and hold the Volume Up and Down buttons at the same time for three seconds.

You'll hear confirmation that the feature has been turned on or off.