About [Select Onchange Submit Form] ไม่เข้าใจจริง ๆ ครับ ช่วยหน่อยนะ

<form action="register_save.php" method="get" name="form_register" onsubmit="return register();">
แผนกวิชา
<select name="depart_fname" onchange="javascript: document.form_register.submit();">
<?
$result_department = mysql_query("select * from department ");
while($record_department = mysql_fetch_array($result_department)){
?>
<option value="<?=$record_department[dep_id];?>"><?=$record_department[dep_name];?></option>
<?}?>
<input type="submit" name="save_register" value=" Save " class="submit">
</select>
</form>