Document a few things that Phusion Passenger does under the hood.

doc/Users guide Apache.txt

@@ -1513,6 +1513,55 @@ try drogus's link:http://github.com/drogus/apache-upload-progress-module/tree/ma
 Apache upload progress module] instead.
 
 
+== Under the hood ==
+Phusion Passenger hides a lot of complexity for the end user (i.e. the web server
+system administrator), but sometimes it is desirable to know what is going on.
+This section describes a few things that Phusion Passenger does under the hood.
+
+=== Static assets serving ===
+Phusion Passenger accelerates serving of static files. This means that, if an URI
+maps to a file that exists, then Phusion Passenger will let Apache serve that file
+directly, without hitting the web application.
+
+Phusion Passenger does all this without the need for any mod_rewrite rules. People
+who are switching from an old Mongrel-based setup might have mod_rewrite rules such
+as these:
+
+------------------------------------------------------------
+# Check whether this request has a corresponding file; if that
+# exists, let Apache serve it, otherwise forward the request to
+# Mongrel.
+RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
+RewriteRule ^(.*)$ balancer://mongrel%{REQUEST_URI} [P,QSA,L]
+------------------------------------------------------------
+
+These kind of mod_rewrite rules are no longer required, and you can safely remove
+them.
+
+=== Page caching support ===
+For each HTTP request, Phusion Passenger will automatically look for a corresponding
+page cache file, and serve that if it exists. It does this by appending ".html" to
+the filename that the URI normally maps to, and checking whether that file exists.
+This check occurs after checking whether the original mapped filename exists (as part
+of static asset serving). All this is done without the need for special mod_rewrite
+rules.
+
+For example, suppose that the browser requests '/foo/bar'.
+
+1. Phusion Passenger will first check whether this URI maps to a static file, i.e.
+   whether the file 'foo/bar' exists in the web application's 'public' directory.
+   If it does then Phusion Passenger will serve this file through Apache immediately.
+2. If that doesn't exist, then Phusion Passenger will check whether the file
+   'foo/bar.html' exists. If it does then Phusion Passenger will serve this file
+   through Apache immediately.
+3. If 'foo/bar.html' doesn't exist either, then Phusion Passenger will forward the
+   request to the underlying web application.
+
+Note that Phusion Passenger's page caching support doesn't work if your web
+application uses a non-standard page cache directory, i.e. if it doesn't cache to
+the 'public' directory. In that case you'll need to use mod_rewrite to serve such
+page cache files.
+
 include::users_guide_snippets/appendix_a_about.txt[]
 
 include::users_guide_snippets/appendix_b_terminology.txt[]