Batuchka is correct...it's when certain IGPs on the motherboards work in conjunction with a low-end GPU of a very specific variety. I believe the motherboards are the 780g, 785g, and even the 880g. The graphics cards that line up are the 26xx series and up to the 3450, I think. I believe it has to do with the stream processors being the same.
The performance boost is good in comparison to the IGP, but I think for the most part hybrid xfire and hybrid SLI have died off quite bit because the idle power for regular 2D rendering is so low in cards like the current 5xxx generation from ATI.