It's for powering two power supplies using only one motherboard.
The corsair 620W can't supply enough stable power to 3 4870 cards and an overclocked i7-920 - so my solution is to use two power supplies for the system.
Originally I soldered some converter cables together, but later in the weekend I found a premade cable with the same purpose.
Here's a pic of the cable:http://opelcc.game-server.cc/albums/Privat/computer/e6400/slides/DSCF2186.JPG
It works by simply adding a second connector attached to the motherboards pins for the 'green wire' and a ground wire ... the green one tells the power supply to turn on and off.