How do PC sound cards cope with headsets of different impedance?
Maxim makes combined codec+amp chips that have this impedance detection feature built-in.

From the MAX9856 datasheet:


  Comprehensive load impedance
  sensing allows the MAX9856 to autodetect
  most common audio and audio/video headset and jack
  plug types


Then, later:


  HPL, HPR, and JACKSNS provide a headset detection
  feature which can both detect the insertion of a jack
  and measure the load impedance.


It later explains in a bit more details the process they use to guess the impedance. Basically, they inject current in a way they hope won't be audible, and check the resulting voltage.

Anyway, even later in the datasheet is described the result you can actually get from this testing in some given register:

BITS | LOAD
00   | 200Ω &lt; load &lt; open
01   | 50Ω &lt; load &lt; 200Ω
10   | 0 &lt; load &lt; 50Ω
11   | Idle state


So you indeed get three specific relevant cases, just like in your advertisement.

I therefore suspect the motherboard just uses this chip (which, frankly, doesn't seem to have amazing performances other than that), or a similar one, and makes a big marketing deal out of it. The designers certainly didn't design some complex, specific circuitry to solve this problem.

But anyway, as JRE said, knowing the impedance of the connected load isn't actually a problem. The only consequence is that the power consumed by the headphones will be different depending on the impedance, so the sound will just seem louder or quieter (and this depends a lot on other factors too, like the efficiency of the speakers). The user will adjust the volume accordingly. This is not supposed to have any significant effect on sound quality anyway, except if you start to saturate the amp output.

So the only advantage this thing could really have is to eventually avoid regularily changing the volume if you use multiple kind of headphones and switching them constantly.