The descent of is the key move.
In the continuation up to , a and b are miai for life.
It is fairly obvious that in order to live, the lone black stone, which White can put in atari, must be defended, in order to keep sufficient eye space. Yet, this has to be done in a way preserving the eye shape. The descent is just the right move for that.
Simply connecting is not enough. is pretty well forcing, and after , Black has six stones on the second line.
Black's move here gives ko in various ways, of which the best one is to start by capturing.
here seemingly abandons the marked Black stone in order to secure life by playing at . However White takes him by surprise and plays at , the vital point now, herself. Whatever Black tries now, even going back at the correct move at a, the exchange of and will prove to cost him life.