Writing an ACPI driver - an introduction

It'll be called when the firmware generates a notification, which is up to the firmware. Since all the driver does is then attach the device, you can only detect that in userspace by looking for the uevent generated when the USB device reattaches. There's no way (with this driver) for userspace to forcibly change the BT status. Doing that would require it to hook into the rfkill layer.