This is the life-saver cheatsheet you need if you're ever going to want to understand complex rewrite rules like the ones generated by the Boost module in Drupal:
http://www.askapache.com/htaccess/mod_rewrite-variables-cheatsheet.html
In particular, in RewriteRule .* - [S=5], the [S=5] rule means "skip the next 5 lines". It can represent a considerable efficiency boost! Same thing can be said for [L] (last rule)
A set of nice examples is also available on the same site: http://www.askapache.com/htaccess/modrewrite-tips-tricks.html