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 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!