What is tri-fire?

[Moodle]

Hi guys, as you can see in the title could someone explain tri-fire . I understand crossfire , its were two of the same gpu's can use each other to boost performance. But I have seen tri-fire with like (for example) a 6970 and two 5850 how does that work? does the 6970 boost the performance of the 5850's ?
,Thanks.
( and if you could link to a website/video about it that would also be great) because I cant find any

 

[Moodle]

Thanks for the quick reply's guys. One more question, to set it up is it just a crossfire bridge between each of the 3 cards ? Thanks.