Auto-propagation means that whenever you save translated segment(s) the system instantly translates all identical source texts anywhere else in the text. A real time saver. You even do not have to push a button, it is all automatic when you save a segment, an entire page, when you switch between pages, etc.
Where is it hidden? Look at the bottom left corner in the editor:
A click on the button lets you switch the tool on or off. "Configure" opens some options: By default, propagation does not replace existing translations but only propagates to untranslated segments.
By the way, you can always propagate "on demand" from a segment's context menu - click the orange icon in a segment's top right corner:
You can even assign a shortcut to this...