Nineteen point trick play
The extension at here is the famous nineteen point trick play (also known as the "eighteen and a half point trick play"). Typical continuations for this joseki would be at a or b: see 3-4 point low approach one-space low pincer press.
If White falls into this trick play, Black will give up nineteen points in the corner (hence the name "nineteen point trick play") but get a huge advantage anyway. Read on!
White happily captures the corner, blissfully unaware that Black is intending to sacrifice the corner.
Tas: Is w3 the best? It seems to me that a would remove a black liberty to avoid black getting b in sente later, And white b would threaten to break the wall.
unkx80: Seems to me that it is business as usual for Black...
What happens after this ( on )?
This seems even worse for white I think. Although the black wall is less perfect, white is running on the second line. Or am I missing something obvious?
shmolex: what happens if white instead connects on the first line with 2? It seems like black doesn't get the wall anymore.
Mef: If I'm reading correctly, instead of playing , black will go for the net immediately, white can only escape by giving black the corner.
shmolex: ah, I see it now. Thanks.
The sequence up to results in a huge success for Black. White has 19 points in the corner, but Black has such a thick wall outside that its value is much bigger than White's corner. Besides, Black a is sente. (The Iron Wall page shows a continuation of this example.)
White can play at to break this trick play. Black must defend at and attacks . The result is favourable for White. The three white stones around form a big bulge.