While there are several ways to toggle Talkback on and off, they generally require multiple steps to enable them or disable them.  Some manufacturers like Samsung and LG have added a feature to Toggle Talkback on or off using a hardware button (similar to what is available on iOS) by triple clicking the home button.  However, if you do not have a phone that supports this there is still a way to have the Talkback on/off toggle feature using a 3rd party software package called Tasker.  You will also need a rooted device.    If you are unfamiliar with what Rooting is, please read the following article from Android Central that explains rooting your android device also consult the first part of our Guide To Installing Custom Roms (From a blind person's perspective)  that explains how to root some of the most common devices from a blindness perspective.

Note that this currently doesn't work with Android 5.0 Lollipop.
Be warned that Tasker is really flexable, but a bit hard to use with Talkback.

Based on some reading of the thread at the bottom of this post you will also need the secure settings app, which is free. You'll also need to enable the system+module (can be done in the secure settings app itself.

The below project file when unzipped and imported into Tasker will toggle talkback when you press the power button twice quickly.   Additionally, this project will make your phone speak the window title when talkback isn't enabled.  This will allow you to have some idea of what is being done on the device when Talkback is disabled.  To use this feature it is necessary to enable the tasker accessibility service.  You can find it in the accessibility settings on your device.

Taskr Project File
Instructions for importing the XML file
First copy the xml file on to your device. I put it in download but it doesn't matter.

1. open tasker, and press ok on the disclaimer and the overview pages. The ok button is located near the top right.
2. open the options menu and choose preferences.
3. un-tick the beginner mode tickbox. It is an unlabeled tickbox just to the right of the words "beginner mode".
4. press back to close settings.
5. find a button at the bottom left which is unlabeled.  It doesn't say anything. this is the default project. Double tap and hold it.
6. choose import from the popup menu.
7. brows to where you saved the xml file. To go up a level press back which is located at the top just to the right of the current folder name. Go back twice to get to the root of your internal storage.
8. press ok on the warning screen.

Now tasker should show a list of the profiles in the project, and there should be a project called tb at the bottom.

Configuring Secure Settings

1. open the secure settings app.
2. There are tabs at the top. Tap the system plus tab.
3. There is a switch below the tabs in the middle of the screen. turn it on.

That should be it. Press the power button twice quickly to toggle talkback.  It wont work the first time.

If you want limited voice feedback about which app is running when talkback is disabled, go to accessibility settings and enable the tasker accessibility service.

Credit to Robbie who put the project together for us & Pitermach who provided the talkback toggle task., see

https://groups.google.com/forum/#!topic/eyes-free/TJQKZvq92k0