Custom VBO action to print selected nodes
In your hook_action_info() implementation, you should set 'aggregate' to TRUE. For example:

function YOURMODULE_action_info() {
  return array(
    'YOURMODULE_your_custom_print_action' =&gt; array(
      'type' =&gt; 'node',
      'label' =&gt; t('Print nodes'),
      'aggregate' =&gt; TRUE,
    ),
  );
}