Can someone explain this new feature? And which Netflix app is it?
Adds background sound support for apps which do not explicitly support it (like Netflix).
End of dypsis's quote
The feature is on the advanced page as a global on/off switch and on the app settings page on the per app right click menus to force it on/off on a per app basis.
Basically some apps like Netflix were designed to mute their audio when not the foreground app which was annoying if you ran them in a window. This feature stops that happening.
Going forward we may default it to on for some well known apps.