The event is signalled by tpl_content() in inc/template.php. You can use this event to output XHTML before and/or after the main Dokuwiki page output and can prevent the default dokuwiki page generation taking place. Output buffering is active during this event, the captured output is passed to the event TPL_CONTENT_DISPLAY.
$data contains the contents of the $ACT variable.
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.