The placement at 1 allows white to capture four black stones in sente or make a break along the first line threatening the life of the whole group.
Attempts and discussion
is a mistake, which loses the whole left-side group. After , White wins the capturing race by one move. Even if Black gives up , he has suffered a great loss.
Black should just give up his four stones and defend his group as a whole. White gains ten points in sente, though: a great result.
Charles How about at ?
Dieter thinks there is more to it.
Black loses about 6 points in gote.
I think that placement gives White sente. HolIgor
If Black is surrounded, at dies. In that case, this ko looks like orthodox play, unless Black is komaster.