Black can't make an eye in the corner.
This is a shape that occurs in many tsumego problems.
Black captures some white stones through snapback.
However, there's no eye in the corner for black.
See also Xuanxuan Qijing Problem309 for another problem with this shape.
As long as is not at , black is alive after .
Notice now the snapback in the corner result in this, which is differenct from the shape before.