Savvy WordPress user here, or at least trying to be. I have the hang of hooks and filters with the code in WP's plugin.php
The only thing I cannot seem to understand is the $merged_filters global. The value for a tag is unset when a filter or action is added.
However, when an action or filter is executed, the $merged_filters[$tag] value is set to "true".
What is the purpose of this?