Access network isolated APIs and databases from Azure Static Web Apps

Thank you, Thomas for insights to securing static app backends, could you please explain a bit more re. this: "When doing so, the backend resource will be configured to only accept requests from the Static Web Apps’ application, to ensure that the access to the backend resource is secured."
How this happens, I don't see any trace of this restriction on function app configured as backend of static app. It still has unrestricted access from the internet for me when I am trying this. Thank you!