Gokyo Shumyo, Section 1, Problem 79 / Solution
The key move here is the descent at , threatening to connect underneath while making shape. After , Black has two eyes: at the circled point, and in the corner.
If Black draws back to , White plays hane at and reduces the black group to an L-Group (White can indeed not play at the marked point). With no weaknesses to exploit, Black is dead.
instead of in the main line, makes sort of a J- Group. The result is ko.
Main line: HashimotoUtaro