eFormOnMailSent, how does it work?

snippet name: show_link
<?php
function test(){
echo 'test';
}
?>

[[show_link]]
[!eForm? &formid=`form1` &to=`[email protected]` &from=`[email protected]` &tpl=`feedback_tpl` &eFormOnMailSent=`test` &report=`formreport` &subject=`filled form` &protectSubmit=`0` !]

function mailSent(&$fields) {
  $fields['return_calc']= "Sent!";
  return true;
}	

[!show_link!]