How does Volumio perform shutdown?

Can anyone advise me on how volumio performs shutdown if initiated through the web UI? Is there a script executed and that can be modified and where to find it?

Reason for asking is that I’m using the RemotePI Board from MSL digital solutions http://www.msldigital.com/pages/support-for-remotepi-board-plus-2015 and want it to switch the Pi off when shutdown from the web UI is chosen. The RemotePi Board needs a special sequence on GPIO 15 to disconnect the power from the Pi.

Many thamks!

BR

Michael