Taisha descent variation
The descent of in the taisha joseki is an alternative to the push of a. It seems to be most common in 3-4 point joseki in which White has played either a three-space low or high pincer (marked points). See 34PointLowApproachThreeSpaceLowPincerTaisha and 34PointLowApproachThreeSpaceHighPincerTaisha for these.
The descent of can also be played without a white stone at one of the circled points, though it is not common. Note that the Taisha is not to be recommended when there is a white stone closer to the corner than the marked points. The result is too good for White in such cases.
This continuation appears in NieWeipingOnGo. creates a target for White so something lighter might be worth considering. White can begin the attack on the Black center stones with a or b.
Notice that in the "normal" Taisha continuation (shown above), White c is exchanged for . In this descent variation, instead is exchanged for . Thus, with respect to the center attack, White has gained a tempo. At the expense of solidifying Black's corner, White gets to initiate the attack on Black's center stones. Locally, this seems to be the main difference between this variation and the more common one.