This is a deviation from the tsuke-nobi joseki (namely, at a,
at b) which is possible for White when she has a ladder-breaker in the opposite corner.[1]
Charles Matthews The important ladder occurs after and
. White should expect this and use a ladder with
, unless White is already strong on the left side in which case White at a may be possible.
The expected sequel is then ,
(because of the proverb take off stones captured in a ladder, at the earliest opportunity
White is considered better.
With here, Black prevents the ladder capture: clearly though White at a next means White can take over the corner for a big exchange.
If White isn't so flexible, and plays here anyway, a difficult crawling battle can arise after
. There are some more ladder tactics available after
(double threat ladder-maker).
The ladder line above is normally good for the side with the favourable ladder, so in practice here on the other side is usually seen. After
, Black at a is the normal joseki, while Black at b leads to a difficult cross-cut fight after White cuts at a.
The usual continuation proceeds with double hane, and White taking the corner. Black has good outside influence and can take more with Black c, White d.
[1] This should just be, when White has a favourable ladder. Charles