Add documentaion about hard focus mode (issue
@@ -27,7 +27,26 @@ and ``en.wikipedia.org``. Links to any other domains will be ignored.
Hard-focus vs. Soft-focus
-------------------------
-TODO
+
+The focus mode (hard vs. soft) is another way to prune the search space, i.e,
+discard links that will not lead to relevant pages. Relevant pages tend to
+cluster in `connected components
+<https://en.wikipedia.org/wiki/Connected_component_(graph_theory)>`_,
+therefore the crawler can ignore all links from irrelevant pages to reduce
+the amount of links that should be considered for crawling.
+In "hard-focus mode", the crawler will ignore all links from irrelevant pages.
+In "soft-focus mode", the crawler will not ignore links from irrelevant pages,
+and will rely solely on the link classifier to define which links should be
+followed and their priority. The hard-focus mode can be enabled (or disabled)
+using the following setting in ``ache.yml``:
+
+.. code:: yaml
+
+ target_storage.hard_focus: true
+
+When the hard focus mode is disabled, the number of discovered links will grow
+quickly, so the use of a link classifier (described bellow) is highly recommended
+to define the priority that links should be crawled.
Link Classifiers
----------------
