Tsumego From Games 4

Black to kill the upper right corner  

The attention is drawn to the upper right corner. The White group is trying to live thus cutting the top center group from the upper right corner. Had I calculated it a bit earlier, I'd have left this as aji. Black can kill this group.

Flying saucer shape  

This is a standard shape, the flying saucer shape, and the hane at 1 kills.

Mistake  

My opponent played 1 and 3. If I continued with a, Black b would still have killed me. If White b, then Black c kills. But, there was a tesuji to save the white group.

Tesuji  

White 1 threatens the cut at a and at the same time prevents b in the previous diagram.

The threat  

If Black blocks, he loses some stones at a or b.

Caution  

If Black connects his marked stone, White first connects, then captures the stones at a or b.

More Caution  

If Black connects, White lives with 5. White 5 at a would be a big mistake, for Black b will make miai of 5 and c to kill.

Continuation  

So Black protected at 2 and White went on to live in the corner with 3. Next, if Black a, White b.

Continuation  

This seems to be a ko -- if white connects with 4 at 1, black at a makes a double damezumari. Thus, white 4 must be at a and black starts the ko.

Continuation  

unkx80: Unfortunately your argument doesn't hold water. lives.

Continuation  

inoru: However, allows , which renders white dead in this corner. For white to live, you must remember the rule of ko.

JoelR: This comment does not make sense. is self-atari, White captures (more than) three with to make an eye. After White fills , there is no ko in this area.

Continuation  

(inoru?:) Doing so forms this shape, and white is successfully alive.

JoelR: On the other hand, if White plays here, then Black can recapture the ko at , and White is not assured life.