Gote seki / solution 1

White komaster, ko  

If White is komaster, she can make ko and win it, for 5 points of territory, in exchange for a Black play elsewhere.

White komaster, no ko  

Normally Black should make this play early, when a Black play elsewhere would gain more than 2½ points, so that even if White is komaster White cannot afford to make and win the ko.

Tenuki is wrong  

This is no seki, it's bent four in the corner.