The solution includes a ishi no shita tesuji. After White 1, a is a vital point.
Black sacrifices two stones, hoping to destroy the eye by giving atari with 6 at 4.
White makes the surprising move at 2, allowing Black to capture four stones !
In the space created by the four captured stones, White can recapture two stones.
Obviously, White mustn't answer 2 at 3 here.
If White hastily answers the atari by (temporarily) saving her stones with 2, Black 3 kills.
If White fails to make the four-stone sacrifice, and connects at 5 here, then Black 6 takes three stones, and there is not enough room to play "underneath".
White 1 makes one eye in the corner. Black 2 makes miai of a and b to take away the other eye.
Black 2 here would allow White to live in ko.
Go to Problem48