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According to this interview of Tim Sweeney, their technique is called SVOGI (Sparse Voxel Octree Global Illumination) and... it is based on the voxel cone-tracing technique we published last year at Pacific Graphics ! :-D
That's really awesome to see research results being used so quickly in a game engine ! Beyond the paper, you can find more details about our voxel cone-tracing approach in my GTC 2012 talk, my Siggraph talk, as well as in my Ph.D thesis on GigaVoxels.