The "the_content_feed" filter is used to filter the content of the post after it is retrieved from the database and filtered by "the_content" and before it is sent to RSS reader (or browser).
A plugin can register as a feed filter with the code:
<?php add_filter( "the_content_feed", "plugin_function_name" ) ?>
Example of filter function:
/**
* @param $content Content of post
* @return string
*/
function plugin_function_name($content)
{
$content .= 'Total '.str_word_count($content).' words';
return $content;
}
NOTE: that the filter function the plugin defines must return the content after it is finished processing, or feed readers will see a blank item and other plugins also filtering the feed content may generate errors.