threatens to make life in the corner, while creating approaches to the existing liberties. If Black threatens the connect-and-die at 3, White simply blocks, and is a necessary approach move, allowing White to win the semeai (which then stands at 2 liberties each).
If Black plays at directly here, White should answer at 3. Black is captured. leaves smaller ko threats than any other way to capture. Should Black play a ko threat that White ignores, the damage is smaller.
looks like the right way to exploit the shape, by analogy with problem 4, but after , the best White can do is to set up a ko.