The comment is that lives locally, while a play by white at a instead makes one eye and aims to connect out to the right. Can white make unconditional life in the corner after ?
Dave: is the correct response to and White is dead in the corner.
Dave: This is a more sensible try by White.
Dave: If Black tries here, results in ko.
Dave: here gives White life.
Dave: Oops, alive here too!
Dave: This is the killer. If , the placement at prevents White from making two eyes.
Dave: In this ko, White has a big threat to damage Black's territory by capturing at 1. If Black fills , White gets a live shape by playing the hane and connection at , .
Dave: White can also make ko like this, however...
pwaldron: There is some danger to this sequence. See below.
Dave: When Black connects to protect the top side. White still only has ko inside like this with no threats against the outside. So this approach is inferior to the standard line that Bill cites above.
Dave: If White moves in the previous diagram to here, it is still ko up to . All my comments above are based on the "Shikatsu no Kihon II" by the Igo Henshuubu.
pwaldron: Dave pointed out that White has another ko, but this one is potentially dangerous. Black has the tesuji at 1 to go for a kill. In this particular situation, White may have counterplay because moves such as 'a', 'b' or 'c' may allow him to threaten escape or life.
That being said, if White is embarking on a direct corner invasion, it may be in the middle of a Black moyo and escaping may not do any good.
Dave: By the way, demonstating that the descent at below is inferior to Bill's line is not trivial. We leave this as an exercise for the reader... :-)
Bill: I seem to recall this being joseki.