Hi
What you will most likely have to do is to define a USER event associated with your relay switch in order to launch the subvi.
here's an example on NI website;
http://sine.ni.com/apps/we/niepd_web_display.DISPLAY_EPD4?p_guid=E0CA7D39AD4B54DDE034080020E74861&p_node=DZ52054&p_submitted=N&p_rank=&p_answer=&p_source=External
Regards,
Cyril Bouton
Active LabVIEW Developper