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!
The trick play sequence
and
are is taken in by the trick play.
before
is another way to lure White into it but her correct response is different.
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.
(KataGo thinks White loses 12 points in this sequence)
Refutations
Instead of in the continuation diagram, White should play
here. While Black still gets a squeeze, White can take sente here.
In the conclusion diagram covering at a is not the best option. Rather White should turn at and allow a capture two recapture one at a.
White can dodge this line of play with this calm variation. KataGo advises an even stronger resistance: instead of , white can turn with
at a followed by
and a pincer with
at b.
In this variation, White dodges with a hanging connection, Black forces before connecting and attacks the marked stone.
See also topic:1097 for more discussion on this trick play, as well as a tewari analysis of it. Another tewari analysis of this trick play can be found at Tewari Example 8.
See also Nick Sibicky's video discussing it: https://youtu.be/8BxToud-Wek?t=3473s &
https://youtu.be/dRgoZ7BNjZo?t=102s
See also Ryan Li's video discussing it: https://youtu.be/7dXWFlQA2P4?t=207s