White's four on row 8 has exactly one completion point — a gap four, so there is no second square to defend. A normal block stops the five. A great block stops the five AND builds your own four at the same instant, because the completion point happens to sit on your diagonal too. One stone, two jobs: white's win is cancelled and black's own four now forces white instead. Whenever the forced block point and your own shape share a square, that square is the whole game.
Finish this step to move on.