When White is using the two-space cap to attack with, Black has this attach-extend method to make shape. This may not be appropriate, though.
This play for Black has been taught for dealing with the two-stone cap in high-handicap games. It doesn't mean it is always useful, though. This sequence has been responsible for some joseki as a source of bad habits mistakes.
According to Kim, Black must at least take this bulge point in the centre, in order to fight on equal terms.
In the background there is this possibility for connection along the edge; so Black should play in confident style.