Shoulder invasion of mixed three space extension
One of the three-space extension invasions is a shoulder hit on the lower stone of a mixed-height extension.
The shoulder move is one of the more common moves against a mixed three space extension, Black a, b or tenuki is to be expected.
In practice it is played as ladder breaker, actual or preemptive, as reduction, as focal play in the context of larger frameworks, as a forcing exchange to strengthen a weak group on the outside, or in a fashion similar to a leaning attack.
If intended as a reduction White moves at c or d, which are commonly answered at e and f, respectively, are more often seen than the shoulder hit.
Slarty: on an open board, Black can probably be pleased to answer at . It's then not possible to separate the black stones without acquiring a heavy shape. Not sure if there is a SL page for normal shoulder hit side josekis.
tapir: Well. This is practically never played on open boards, hence midgame. If separating is the main intention I wouldn't start with , which offers Black options to connect. In this variation if Black does not connect, White has an option to separate, that does not need belittling. White at a later does not look like bad shape to me. (Transposition of a midgame joseki when Black invades White, afaik.) Finally, SL coverage of midgame joseki is pretty bad. Reduction techniques was a good start for reductions, but the individual pages are lacking content.
Slarty: It's true if somehow gets Black to play elsewhere, W has an option to cut. Probably the point to the left of a. If a, B can respond at the point to the right of a.
When Black can't afford being cut off from a weak group in the center might be necessary, which allows White to break in after e.g. at a.
This framework building was followed by an invasion at a.