Don't descend, connect
Black to play and settle the top side shape.
This situation comes from the 4-4 point, double low approach.
Instead, we play , which is sente. White should not omit or a black play there might be sente. The other advantage of playing is that it induces a cutting point at a.
Phelan: This seems to be a case of OneTwoThree.
Arieh Question - The cutting point at a seems to completely undermine the purpose of settling the shape. I'm only a beginner, but it seems to be me that if white plays a here, black is in very bad shape, so the gote play in the previous diagram is preferable. Also, I don't think white has to play 4 immediately. White can cut at a in sente, protect the cutting stone by extending and threatening white's triangle above and then play 4.
Bill: If cuts, can catch it in a ladder. If White tries to escape with , continues the ladder. On an empty board White will eventually be captured. (It is possible that a White stone will break the ladder. But unless we see such a stone in a diagram, the convention is that it does not exist.)