Look for a cell that contains only two possible pencil numbers:

In the rows and columns of this cell, look for two other cells that each contain one of the numbers in the first cell and also share a second number. These three cells form three corners of a square.

In this example, the squares have an 8 and 6, and share a 9.

Find the square that forms the 4th corner of this square. As the two adjacent cells share a possible number that number must appear in one of them, this number cannot appear in this 4th corner, so can be eliminated. 

So here, the 9 can be removed as a possibilty for this cell: