3-4 point high approach, double contact, push, cut
This is a classic complex joseki, involving an important ladder. has also been played at a in some games by top players.
After these plays the ladder formed by must be broken, that is the ladder must be good for White, of course. Otherwise must be moved to the location of . Black must check that this ladder is good for him before cutting at (here marked as ) in the previous diagram; so that this sequence doesn't appear in games between players who know what they are doing.
When the ladder is good for Black, which we can therefore presume, is best, when and set up miai: either we have Black a, White b, Black c which captures the corner, or Black makes the severe cut at d.
Therefore White is in some trouble. There are quite a number of variations in pro games, this being an example of an unexplored joseki. Recently it seems that Cho hun-hyeon is willing to play the white side of this position: perhaps White's loss in the corner can be compensated for by pace, in his style.