and
force
- else Black plays there and starts a big ko for kill. Later, Black can start a low risk ko (so-called "flower viewing ko") for seki by throwing-in at a.
White will eventually need two inside moves to avoid the seki above. The result is 5 points of territory (3 vacant points + 2 prisoners) - the same as shown in the Exercise diagram. So Black has no risks - he can start the ko only when the situation is favorable.