UI Guidelines : Left Navigation button should be a previous button
In an official Apple HIG there's the statement:

When the user goes to a new level in a navigation hierarchy, two things should happen:

The navigation bar title should change to the new level’s title, if appropriate.
A back button should appear in the left end of the bar; it can be labeled with the previous level’s title if it adds value.


I do not recall any information about this in WWDC videos.