The opponent has a very dangerous threat: on row 4, X’s have three in a row in columns 2, 3, and 4. Both column 1 and column 5, if available, would allow them to drop a piece and win next turn. While I can only block one threat at a time, I must at least block one of them. I choose to block column 1 by dropping my piece there, even though it’s likely that the opponent can win by the alternate threat. This is the best defensive move in a losing position, so I drop in column 1 to block one of the immediate winning moves.