Catch the selection of the values in select/drop-down-box prior to the node submission
You might be looking for hook_node_presave()


  Act on a node being inserted or updated.
  
  This hook is invoked from node_save() before the node is saved to the
  database.


To attach a custom submit handler, create a custom module and do something like this:

function my_module_form_alter(&amp;$form, $form_state, $form_id)
{
  if($form_id == 'form_abc')
  {
    $form['#submit'][] = "my_module_form_submit_handler";
  }
}

function my_module_form_submit_handler($form, &amp;$form_state)
{
  drupal_set_message("The form is being submitted, do some extra stuff now...");
}