Assume these plays already made.
Avatar DJ Flux By the way, I do not know whether this has been mentioned elsewhere, but Black has a way to stop White from continuing to the right:
FredK: This sequence is discussed in Sonoda (p. 210). He dislikes it because it leaves White both alive and not sealed in. He gives several pages of variations more like the one discussed by Dieter, below, which leave Black squashed into a second line position.
That will mean that White has a chance to play at a here, so there must be a reason connected with the upper side for Black to play this way.
danoontje The variant is like this:
here looks like the modern style, but I don't know if it is good; it was once suggested to me by a 6 dan.
Perhaps there are occasions when this line is good, for example if invading at san-san would give White too much influence, but generally sub-dan players like this variation because it can lead to complicated fighting. Even so, it doesn't obtain much territory in itself, and that fighting often favours White. Things might go something like this:
The honte is for Black to play at , to correct the thin position; this line shows what happens without that play.
Please correct me, because I'm diagramming from memory and I haven't really checked its validity lately.
Tamsin I have tried this, but Black usually plays like this:
If White saves , then Black can connect beneath with a much better result than in the previous diagram. If White blocks, Black captures and a tricky situation emerges:
Black's stones are not easy to attack because White's group is also weak. Have I missed something?
Charles It ought to be better to play at a, anyway: stronger towards the corner.
Moved here from 4-4 point low approach, two-space high pincer with side stone.
SnotNose: Not sure if pros play this nowadays (or ever) but it is often played by amateurs (at least up to 1 dan).
SnotNose: Black has this annoying little follow-up that has led to some ugly play in the games I've seen.
Other variations are possible ( at a, for example) but all lead to fighting and turns out to be very handy in the fight.
Alex Weldon: After this, White at b looks interesting to me, taking advantage of Black's shortage of liberties.
SnotNose: That's exactly what I thought too when I first saw this sequence (ugly shape, aji keshi, etc.). However, Guo Juan 5P has recommended it to some attendees of the recent US Go Congress. So, for that reason alone it is hard to dismiss too easily. Moreover, if black is a strong fighter, this can be an advantageous way to play. I think the relevant question is: is the fight a fair one? If it is at least fair or, due to board position or relative strength of players, tilts in Black's direction, then this may be an okay way to play, no?
Other thoughts: bad shape isn't always bad. The aji keshi is due to the fact that Black could have cut White off from progressing across the top. However, if the position is such that Black wouldn't benefit from limitting White at the top, then is it really aji keshi?
That completes my thoughts on this sequence. I won't defend it any further.