This nifty little option outputs to stderr what the header graph looks like!
It is a bit like a table of contents in your terminal, for when you need to have a look at the outline of the document to decide where to place a new header, but are not in the mood to open a browser or use the browser editor with preview.
Sample output excerpt for this document:
= h1  ourbigbook
== h2 1 quick-start
== h2 2 design-goals
=== h3 2.1 saner
=== h3 2.2 more-powerful
== h2 3 paragraphs
== h2 4 links
This option can also serve as a debug tool for header tree related features (confession: that was its original motivation!).