function YOURMODULE_nodeapi(&$node, $op, $teaser, $page) { 
switch ($op) { 
   case 'rss item': 
     if ($node->type=="lesson") { 
       return array( 
         array( 
           'key' => 'enclosure', 
           'attributes' => array( 
             'url' => $node->field_skypecast_url[0]['value'], 
             'type' => 'application/x-bittorrent' 
           ) 
         ) 
     ); 
   } 
} 
}