You can use the Volume buttons to turn TalkBack on or off.

To enable the shortcut:

1. Go to Settings > Accessibility.
2. Touch TalkBack > Settings.
3. Under Miscellaneous, turn TalkBack suspend and resume shortcut on .
4. Touch Resume from suspend to select whether readout resumes when you turn the screen on, when the lock screen is shown, or only from the notification.

To use the shortcut:

When TalkBack is on, press and hold the Power button, then with two fingers, touch & hold the screen.

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