All you need to know about Carpenter's Square - 22
We already learned that attacking at 'a' from outside is not working. What else?
is urgent. If connects, after forcing moves and , simply reduces the eyespace to kill.
If , is a good move. White again can kill straight-forwardly. at 'a' is useless too.
is an alternative way to kill. After , 'a' and 'b' are miai points.
gives away the key point to black. There is no way to kill this corner any more.
Conclusion: This shape is quite simple. As long as white gets the key 2-2 point, black has no chance.