maximizes the eyespace and threatens to make a separate eye at a, requiring . Then makes a seki life.
Neither player wants to play at a or b. If Black plays one of them, White makes a bulky five by connecting at c. White plays at a or b can be captured later. So both groups are alive in seki.
The other descend is a decisive mistake. White reduces the eyespace with and kills by making the bulky five. Defend the longer side where you can make a separate eye.