Not a "little known" feature, it is the feature that lets you plug your head phones into any of the 6 jacks that it looks like it fits in on the back of your computer and the little pop up says "Are those headphones you just plugged in?" you say yes and that jack is retasked as as headphone jack even though it was the subwoofer output according to the legend on the computer plate.

Its a "feature" that they have sold to a lot of manufacturers as an ease of use thing for people who are frustrated because they plugged in their headphones but get no sound.

Do you get a corresponding "Is that a speaker you just plugged in?" popup if you plug a speaker into the microphone port?

How does the computer know whether you've plugged a speaker or a mic into the port?

Microphones have a higher impedance than speakers. A moderately clever circuit can differentiate the two.

Mics have lower impedance than active speakers, which is the kind you plug into those jacks.

Reads like a back door to me.

This feature allows you to plug any audio device (input or output) into any audio jack on the computer and it will JustWorkTM.

Even if the jacks are color coded or annotated, these day they are actually universal.

So it's more of an accidental backdoor.

It's the very, very old contradiction between convenience/features on one hand, and security on the other.

The more convenient something is, and the more features it has, the easier it is to hijack it.

They aren't fully identical, though. On my motherboard with some Realtek 8xx chip, plugging headphones into front panel jack, green jack, black jack and everything else produces progressively worse quality. Mainly bass suffers and only when driving headphones (no problem driving amps) so either these outputs have lower power capacity on the chip or DC blocking capacitors on the motherboard are smaller.

It can be very useful. I know about this feature due to breaking the headphone jack on a laptop and turning the input into an output to replace it.

HDAJackRetask from alsa-tools is the tool for this on Linux.

No, a feature. There are many valid uses for that sort of flexibility.