increases the liberties to three for the white group. is a must-know technique in capturing races - Black needs an additional approach move at before being able to play at a.
After this , the variations get a little complicated, but white can kill without ko. (Black is trying to set up something like a tombstone squeeze, but it doesn't work in this context.)
If Black is allowed to play at , the white group dies. It doesn't matter whether White played at or at a, b, c or d.