[unwind]

I understood is as the stub was plugged into the input to avoid having the input automatically ignored due to not having a device connected.

I don't know (no experience) if Macs' analog ports were this clever, but considering the hardware to detect presence in the port is trivial, it wouldn't surprise me so that's how I interpreted it.

[bradleyland]

I've seen a lot of Windows hardware drivers that do this. They detect when you've plugged a device in to an audio port and change the config to accommodate. OS X doesn't do this. If you configure Line In as the default input, the OS will monitor that port, regardless of its state. The only adjustment OS X makes is to remember input volumes based on plug state. So for example, if you plug in a set of headphones, the volume will be adjusted to the level that was set when headphones were last plugged in. When you unplug, the volume level is reverted to the state it was in prior to plugging in headphones.