Three cells have exactly two candidates, sharing three candidates in an XY, XZ, YZ form. The XY cell shares different groups with the XZ and YZ cells. The Z candidate can be eliminated from cells in the group with both XZ and YZ.
Like many of the advanced strategies, this is quite difficult to put into memorable terms. Study the following,
The red cells form the XY wing. X = 1, Y = 3, Z = 4. XY = 13, XZ = 34, YZ = 14. The 4 can be eliminated from the green-bordered cells, these are the cells that form groups with both XZ and YZ.
It doesn't matter whether XY is equal to 1 or to 3. The 4 will be in XZ or YZ. That means it cannot be in the green-bordered cells.