The tesuji here is . After , Black can't create a dead shape with a, because he is in shortage of liberties (this is oshitsubushi). Note what a difference a black stone on the marked point would make.
White has to be careful to answer at and not at . The strength of is precisely that it already prevents the connection underneath.
Answering at is the only move. It appears as if White can destroy the eye after this sequence by throwing in again at , but White will simply connect at a.
Just to clarify: If Black tries to leave White with a big eye by connecting at a (almost filling the enclosed space with the dead shape of three in a line), White runs to b and claims a second eye at c.
How should White then respond if Black tries playing at d? That won't work for Black; White runs along the second line and if Black follows, eventually White can block on the first line and the resulting capture will give the second eye (at least). I don't think playing at e directly will work; seems like Black sets up a ko that way. White could probably also escape in this sort of situation. -- KarlKnechtel
There are many ways for White to go wrong. Let's assume she takes as the obvious move, but fails to see the tesuji of in the main line.
Black kills with the same hane.
xela: Yes, I think at a works.
Why not the simple descent of ? This makes miai of a and b, no?
xela: Black won't be kind enough to reply at , but will play c instead.