Gokyo Shumyo, Section 1, Problem 69 / Solution

Main line  

Black starts at the 1-2 point. Usually, White keeps this shape to one eye, but here there is aji in . The key move is . After , Black can either capture four stones in shortage of liberties at a, or make two eyes at b.

Failure at 1  

Because of the presence of , doesn't work. and are miai, and so are a and b.

Failure at 5  

If here, White destroys his eye with .

Failure at 5  

Now and are miai to take the eye away.

Failure (ii) at 5  

here is not sufficiently forcing on White's four stones. White takes the time to create a killing shape with . (unkx80 has written specifically about this shape at rectangular six in the corner.)