FSP have been working on what you'd think to be the perfect solution to making sure your power hungry multi-GPU based setup is getting continuous uninterupted power for optimal reliability and performance. The Booster X3 offers a seperate dedicated 12V line from a 5.25" based unit that slides neatly into an expansion bay.
Hexus however have a couple of negative feelings about it, as mentioned here.
The idea behind FSP's Booster X3 is that when users get an urge for some multi-GPU action, but fear their existing PSU won't be up for it, they can use this as a secondary PSU to meet to satisfy a pair of power hungry graphics cards.
From the specs of the Booster X3, it only serves to provide 12V. We're guessing, then, that it simply possesses a pair of PCIe power connectors for use with two graphics cards. However, FSP could have implemented it differently. Unfortunately their press release doesn't detail this. The specs also say there's a ripple of 150V, so we're hoping that actually means mV.