In the page on the one-space low pincer, it is mentioned that this corner is ko if Black plays first. There are two ways to start this ko, but one is clearly better than the other.
The 'wrong' way to start a ko is by this sequence. If Black wins the ko in the conrer, White dies. If White wins it, she lives.
The 'correct' way to start ko is with and
here. This ko is more pleasant for Black than the previous one for two reasons: