Simply put: Every pair of dynamic driver headphones or earbuds is a microphone! Should you ever find yourself in a situation where enemy soldiers have pinned you down and all you have is your trusty pair of headphones and a radio with no mic, fear not my Macgyvering friend, you can now safely call for backup!
To quote Mental Floss' description of how this works: The diaphragm of the driver of the headphones is moved by the molecules of air that make up a soundwave. This driver is attached to a coil of wire (voice coil) which rests between a magnet. As the coil moves through the magnetic field, a current is produced.
In short, a speaker is just a microphone in reverse. Instead of the soundwave moving the diaphragm it is the magnet moving it to produce sound.
Since headphones are not designed to be microphones the quality is certainly not going to be great... or good... or even "Turtle Beach." But hey, those enemy soldiers should have taken your sweet cans from you before locking you in this radio shed.
So how does it sound!? Lets do some tests! The following results are raw audio recorded in Audacity and exported to MP3.
Audio Technica R70x Open Back Headphones
Audio Technica M50x Closed Back Headphones
In short, it's no ModMic, but science is still awesome.