3-4 point high approach low extension, trick play
After , is a normal response. however is not normal, this move is normally played at a. is the only move, after which black plays . With and , black is trying to trick white.
The normal continuation for white is to play a. White b is a mistake which will result in a bad position (See variations under 'White atari').
After white extends with , black normally plays . Alternatively, black can try to brew up trouble by playing c (See variations under 'White descends, black hane up')
The correct way to play for white is to give atari at a (see 3-4 point high approach low extension), but many players will be tempted to cut at b. This is a mistake.
After this sequence, may players will feel satisfied that they have captured the marked stone . In reality, they have been tricked.
One way for black to continue is to play like this. White gets only 4 or 5 points in the corner, while her marked stones () are cut off. Black is strong on both sides, so this result is terrible for white.
Dieter: AI prefer this variation, getting the corner in sente and the aji of the marked stones.
Black can also continue with . This sets up another trick because the corner is not 100% alive yet. If, for example, white considers the corner alive and plays through . Black can kill the corner.
Herman: @Dieter, how does AI feel about this? Extending is, IMO, the main line for black. The atari in the previous diagram feels like a thank you move.
After , white cannot play at a due to shortage of liberties
Considering the above, white should play 1 to defend the corner. Now the p-q-r threat is gone, because the white capture at s then creates 2 liberties (r, t) instead of damezumari.
Herman: @Dieter: Specifically, this is considered the "correct" continuation after extending and now black gets sente (or he can use it to spend a move to encircle white's central stones)
The classification of above as "trick" or "obvious threat" is in the eye of the beholder.
After , the players are in for a complicated fight. White can avoid that fight by playing a, but playing a comprises a small loss for white.
After black captures the stone at 3-3, white can only give atari with . After black defends, white has two cutting points. White will defend the upper cutting point at , after which black cuts with . With and , white sacrifices another stone and sets up a ladder. If the ladder is bad for white, this is a disaster, but even if it is good, black is very happy. Black gets a big corner in sente and can look forward to play a ladder breaker later on.
When White extends with , Katago postpones life in the corner, stressing central influence.
When connects at the circled point, Katago sees possible continuations at a, b or c, still not thinking about living in the corner.
Herman: Interesting. Killing the corner is a 20 point swing, but apparently the outside is bigger than that. Thanks for the insight!