here, giving an example of the staircase shape, is one of the most surprising joseki plays. It is seen also when
is played at a.
HolIgor: One should remember that is ladder dependent. If Black has a ladder breaker in the opposite corner, White could end up with a very bad result.
SnotNose Let's be explicit. Black to play. Where is the ladder?
Black needs another protective play in the corner. White might sneak a ladder break in at some point along the way before black can play a, b, or c. The corner is large for Black and White has played somewhat inconsistently, first diving into the corner and then sacrificing a lot to get a wall on the outside. White's outside wall is not so thick, along with a black play at c might give white some things to worry about later.
There are many possible lines in the books, but in this case a standard continuation seems to be current, as shown here. is to avoid being shut in. In this case playing atari in the staircase formation turns out better for Black than connecting solidly at 4.
With on the fourth line, there are some trick plays possible if Black connects. Top players have continued as shown with Black, playing for central influence.
Sazn: Sometimes following what a pro does is not good, since you may not know why or the basics of it.
In a jeongseok book as a punishing move
This will make White run, since he/she lacks a base.
Black a after .
Charles The number of examples in pro games is small - this way might be suitable for kyu players. But the previous line is more common, it seems.
Sazn I believe there is a follow up dealing with sacrificing stones...