Related proverb: strange things happen at the one-two point - in this case you need to know which of the two 1-2 points, of course.
If White next blocks at a, Black makes a big eye at b. And if White draws out at b, Black enlarges the eye with a.
NB: if White has a stone on one of the circled points, her move at b suddenly works and Black is dead, although a lot of aji remains, because he can break out into the centre (in this position).
Finally, what about making the corner eyespace as big as possible, with here? White plays hane at , threatening to capture three stones. Black has to make a solid connection. This is as in Gokyo Shumyo, Section 1, Problem 1, but with a white stone at , and Black is dead.
unkx80: It might be better to have block on the outside.