and
sets up the under the stones tesuji.
But the throw-in at is also a tesuji to circumvent the under the stones tesuji.
The solution is a ko, with White taking the ko first.
Things get easy when descends, all Black has to do is to continue with the original plan.
This is the standard end result of the under the stones tesuji.
Suppose and
proceeds with the under the stones tesuji...
White lives after , since Black cannot approach at a.