(Referenced by 509)

: ( 550)
If White gives atari first, Black has not lost any liberties, and will not connect. Instead, Black secures his top left corner with , and prevents the White endgame there, so gaining more points than he has lost by not connecting.

: (108 554)
Black cannot connect his stone in atari. Otherwise, he will lose a decisive liberty.

:
After the sequence from to , White has to choose the Capture Variation, to realize her (temporary) territorial gain in the top right corner.

We assume that the final endgame on the left side will follow the sequence shown here.

In the top right corner, White gained seven points - five point of territory (/), and two prisoners ().
In the upper left, Black gained three points of territory (), on the upper side, White lost one point of territory (), and, on the left side, she did not capture two Black stones (). When these all are combined, Black gained eight points.

The result of the Capture Variation (which ends with a Black win by six points) is one point worse for White than before. And, remember that the correct Semeai Variation (Black wins by three points) would have been three points better for White. So she made the wrong choice, when she did not play the correct move on the first line, threatening to cut Black apart.