Low invasion of fourth line three space extension
One of the three-space extension invasions is below the centre of a fourth line extension.
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Black's basic answer is to play and link up the black stones, inviting white to make life along the edge. What happens in reality depends a lot on surrounding conditions. The stronger Black's stones, the less valuable it is to connect them, in which case he will more likely allow White to jump out or even try to kill it outright.
Here's a kind of middle game joseki, arising from a san ren sei
When invades, there is room to both sides, so Black rather connects. If Black can block thanks to the presence of the marked stone.
Although White's invasion has succeeded, it's with small life and the upper half of Black's original sphere of influence is intact, even reinforced. The skirmish has also slightly damaged White's potential at the bottom. Black comes out with sente, so this is an acceptable result.