White plays on the leg side, on the second line. White lives up to . If next Black a...
Bending in from the side without the leg leads to ko. If Black wins the ko, he almost fills White's eyespace with a bulky five. The points a and b are miai for that purpose, and thus serve as a ko-threat for White:
is a ko threat. If fills the ko, makes seki.
White must win the ko to be able to approach at a.