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A Canon patent shows how by replacing the sub mirrors with laminated diffraction gratings, the normal angle of relflection can be altered, adding or subtracting to the usual angle. In the example of the 5Dmk4 this is used to expand the vertical coverage of the AF sensor array.
As with a lot of Canon DSLRs, there are subtle advancements that will make using the tool better. Increased AF point spread is something a lot of photographers have wished for, and I think a lot of photographers will welcome once they have it.