On an android, you can assign physical buttons to activate TalkBack on and off. You can also add other accessibility features to the same buttons. You can add magnification for example using the same button shortcut.
When you click those buttons, a list of all accessibility features will appear on your screen and you will just have to choose which one you want to use.
If you have TalkBack already activated then clicking on the accessibility features on that list will describe them to you before selection.
In real life application -I assigned TalkBack, magnification, screen invertion to the up and down volume button pressed together.