I came across it while solving problems (again). It took me quite a while to get through the problem. I will try to not bore you by discussing every detour I took. But: One of the detours towards the solution led me to the bent four. And that is the one I want to talk about.
Here is the tsumego. I will also present the solution. So do not expand carelessly, if you want to solve it at your own.
|Diagram 2 - Black 5 on D19|
First, let's quickly outrule a wrong variation. It is rather tempting to start with an atari. In this case this leads to white being alive, though. So that couldn't be it. I figured that one out on my own and I'm rather proud on it ;)
I'm glad I followed up at my dissatisfaction. It was what led me to what is coming now. However, anything from here on wasn't in my book - so take it in with care. I also concentrate on the variation that leads to the bent 4 in the corner.
EDIT: If I see it right, white only has the opportunity to force a ko, as long as black plays tenuki and does not close the last outside liberty. Otherwise it is seki.
So the question is: If this variation also leads to a ko, why does my book not talk about it? It took me some time, but preparing the diagrams for this post I finally got it: It is a bend four in the corner shape. So it is dead. But: Why is it dead? Lets take another look at diagram 6!
Black has absolutly no reason to play A as long as there are any ko-threats left on the board. He could easily wait until the very end of yose and then kill the stones by starting a ko white can not win. If white tries to approach, she dies. For that reason the 'bent four in the corner''-shape is considered to be dead.
It was just difficult for me to stop. In essence, black kills by playing tenuki.
I have read about it quite some time ago, now I got it and will not forget again. Nice!