Attachment on the second line
is a solid line of play. Black gets additional territory and White gets solidity facing the right hand side.
This works well for White when White has stones to the right at a or b. White stones closer than that would yield an overconcentrated shape.
Alex Weldon: If the White stone is much closer (say at c, especially if the ladder towards the upper right is good), or if Black is playing this too early in the game, and White is willing to play lightly, it may be possible to omit . This would make the sequence gote for Black.
If White chooses to capture with the hane at , forces White to play to complete the capture of .
Black will then roll over top of White with . Black obtains outside thickness, while White gains some territory. For White to resist by playing there herself is inconsistent with .
Velobici: Such is this 10k's understanding. Please correct my errors and enrich this page.
Charles I saw a 4 dan and a 7 dan discuss this, last year. The 4 dan said he'd automatically play above; the 7 dan commented that the 4 dan needed to change his entire attitude to the game. So, who am I to comment?
Here is natural resistance.
Black has several choices: at a, at b and possibly some ways to fight out with .
If Black avoids complications, up to is expected.
Clearly here White has played in a territorial way, and Black has gained in influence.
Bill: So far, this is one of those discussions that does not take the context into account. (However, see below.) Often this attachment is a sabaki or shinogi play; it may also be a good yose. This second line attachment would be bad on an otherwise empty board.
Bill: For instance, through Black is too low.
Bill: The database guys can check this out, but one of the commonest situations where this attachment occurs is as a defense when separates the two Black stones.
unkx80: This second line attachment is also commonly seen when there is a one-space jump stone, and can be a very big early endgame move.
unkx80: If gives in, then gains a substantial amount of territory. Later, Black can aim for a or b, which may cause to immediately patch up this weakness, allowing Black to gain sente.
unkx80: If resists, then and can push into White's territory on the right. Whether Black's sacrifice of is properly compensated by the breaking in by and depends on the wider context.
This contact play technique is particularly useful for making shape for cramped groups on the side. It is quite common to reinforce the two-space extension.