If in a given row a certain number is determined to reside in a certain 3×3 block; and furthermore, that same number for a given column must also reside in that same 3×3 block, then that number must reside in a cell which is common to both the row and the column in the block in question.
In the example given: For row A the 4 must reside in column 8 or 9 of the upper right 3×3 block. Furthermore, In column 9, the 4 must also reside in row A, B, or C of that same upper right 3×3 block. The 4 must therefore occupy cell A9; i.e., the only cell common to both row A and column 9 – or the intersection of the row and column in question.*
Admittedly, I’ve been doing sudoku for over ten years, and I believe I can count on one hand the number of times I’ve seen this pop-up; but here is a genuine example of it, and solving that 4 allowed me to easily finish the rest of the puzzle.
*For this reason, I personally like to refer to this strategy as “Chuck” after the title character of that TV Series. Fans will understand – he’s the ‘intersect’. Besides, it’s a more catchy name for the strategy!
Given values are black.