By dropping an O in column 1, you occupy row4 col1 and form three in a row horizontally in row4 (col1–3) and also set up a diagonal threat through row5 col0, row4 col1, row3 col2, and row2 col3 (which you can complete by playing in column 3 next turn). Since the opponent can block only one threat, this guarantees a win in two moves.