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.
--Dansc
Charles How about at ?
Dieter thinks there is more to it.
Bill: at a is bad. Simply connecting takes sente.
is better than just threatening to take the stones, allowing Black to play at .
I believe that this is orthodox play.
If Black is surrounded, at dies. In that case, this ko looks like orthodox play, unless Black is komaster.