Shoulder invasion of mixed three space extension

Not an invasion  

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.

Alternatives as reduction  

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.

Some possible sequences

Similar to a shoulder hit on an unsupported stone  

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.

Admission  

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.

Ladder breaker

Jiang Weijie (B) vs Won Seong Jin - Chunlan Cup - 2012-12-04  

Framework building

Takemiya Masaki (B) vs. Takao Shinji - 24th Tengen - 1998-07-09  

This framework building was followed by an invasion at a.

Reduction

Cho Hun-hyeon (B) vs. Yi Ch'ang-ho - 38th Kuksu - 1994-09-24  

While it is a reduction at this point, when Black returns later to a White does not answer and it turns into an invasion only after Black b followed by White c. The invasion is sacrificed in the course of fighting for the White center group.