White can't play at the marked point because of shortage of liberties.
After , instead of White can play at a or b to live. (But here in terms of points may be the best move.)
Enlarging the eye space at invites Black to play at the vital point . Now and are miai. After , White has to prevent the capture and take a liberty at a upon which Black takes away the other eye at b.
Connecting at here is symmetric to Failure 4 and is once again countered by playing on the vital point.
TwoSpaceNotchersNotFullyConnected has analysis of a shape that behaves just like this problem; there is some overlap with this SL page.
Here is a similar position:
In this symmetric situation, both legs of the white group have no liberties outside of the eyespace. Now a move at a or at b will be answered by a move at the point diagonally underneath. White is dead. (But compare Problem 23.)
This shape is analysed in its own page on SL: Flying Saucer Shape.