How does devise forward variables to the top of the view - Application layout?

Question

Ruby on Rails

there is a small piece of code that Devise himself recommends putting in the Application layout

%p.notice
      = notice
%p.alert
     = alert

How does the controller get around to forwarding variables so far?
I thought that variables can only be thrown into one view that renders the action, or in the view itself, through the rendering of the partial. This is not true? And where can I learn more about this feature, because it keeps me awake...
PS Or better yet, some magic book, a tutorial on the internal mechanism of rails, or something "Best practices"