Where four cells from two rows and two columns share the same two candidates, candidates can be chosen or eliminated to ensure that the solution to the puzzle is unique.
This strategy can be quite difficult to understand. Look at the puzzle grid below.
The red-bordered cells form a rectangle. If the bottom right cell of the rectangle is not a 1, the 5s and 7s of the other cells could be exchanged without changing the rest of the puzzle. This would give multiple solutions and breaks the rules of Sudoku.
Therefore, the 1 is chosen for the bottom right cell of the rectangle.