4-5 10-3 enclosure
The idea is to play into the complex joseki with , having waiting.
The other direction of extension is rarely used now - White can get a good result invading at , as you can see by applying the joseki in the previous diagram. In this case ends up on a silly point.
Go Seigen did try this a couple of times very early in his career.
Coming in at a is asking for trouble with Black b. White would want to live quickly and avoid the trappy variation.