When Black plays rather than connecting solidly, it gives White the option of
and
to set up two ladders (White at a and White at b). This is a typical crawling fight tactic.
Of course in this case Black may be content to exchange with Black c, White b now.
Another example comes up in the tenuki line of the tsukehiki joseki (see 3-4 point high approach, inside contact, tenuki variation).
Here after the
/
exchange depends on a ladder. More exactly White would now like to play hane at a.
After the cut , it seems that
is a double-threat ladder-maker, setting up White next at a or b. This is mentioned in Get Strong at Joseki volume 1.
What is not mentioned there is the chance of playing as kikashi: after
White doesn't have an immediate way to capture the two
stones. This was seen in a game Ma Xiaochun-Kong Jie 2002-24-22.
This is from BQM 15.
In this case Black is probably happy to exchange the stones for the
(the reason being that Black earlier made a joseki mistake).
What is said on BQM 15 is this.
Suppose is played: then Black should have a good ladder with
- or else not begin this variation.
One should add a little more: here must be answered by White, so that Black has an option of a parallel ladder also. Depending on the precise position in the lower right, this might be important.
Black might have a chance to push once more at a: reading required.