Bluetooth keeps on playing, even if you connect something and play to the AUX input. BT keeps on playing, since it has first priority. If you pause or stop BT play, any source playing to the AUX input will now be heard. If you resume play on the BT source, it will take over, and the AUX source is no longer heard, but will of course keep on playing, since there is no way to send a pause or stop signal to the AUX source via the line cable. All of this means that there is an input selector switch built into the device, controlled by the BT play function.