This play is White's first choice to make life in the corner.
That is easily done if Black now plays a or b. Black may have the intention of playing the hanedashi variation, c. This is a complex but very old joseki.
[1]: Black plays hane outside
If ,
is compulsory: and now each of Black a, b and c is playable (tenuki was tried by Michael Redmond against Otake Hideo in a game 1990-12-01).
If , White is already alive in the corner, and so may play tenuki immediately. If so, Black has a good kikashi at the circle-marked point, and a slower play at the square-marked point.
The idea of playing at goes back to Shusaku. This sente follow-up leaves Black thick here.
There is also the idea of a direct placement: in the corner has been played in a pro game. This idea may go back to Kajiwara.
It is therefore simpler for White to play , though this ends in gote.
The idea of playing instead goes back a long way; and is still seen in pro games. If Black connects at once at the marled point White will end in sente - but has given away futher thickness here. Probably Black waits a little.
[2]: Black's butting play
Instead, here is a little crude as a way to make shape.
is forced; then Black at a leads to a well-known simplifying line - which has been out of fashion at times during recent decades. Black at b is also worth considering.
This sequence has at times been popular amongst pros (finished by Black 11 at the circled point: White has a peep at the squared point that may be played immediately as kikashi). Shortly after its appearance in the Kisei match in 1979 (game 2 on 1979-01-24) played by Ishida Yoshio against Fujisawa Hideyuki, it became unfashionable for two decades.
In fact its status as joseki has often been questioned. By pro standards, Black's play here is rather simple-minded.
On the other hand, this way to play still occurs in pro games.
is also possible here, for strong shape on the upper side. Then
is usual: White can play at the circled point, too.
At this point Black can continue at c, as if this were a pushing battle. In recent games the pro play is more often to cut at the square-marked point, which is a probe.
Assuming and
, Black should be happy with shutting off the open skirt on the top side.
Assuming instead,
is the Cho hun-hyeon style. Assuming
-
, this is a slight improvement on the classic result, just in endgame terms (weakness of the 2-2 point).