zinger: Ok, here's my try. Black can kill white's group with a, so white needs to live. Playing at a works, but does nothing else. also lives, and is a point or two better. But is even better, as it threatens to make a second eye by descending to . If now , white lives with . Perhaps this is an example of force before defending.
Truc: at also works. I think it's the same point wise.
unkx80: here is inferior, because is sente (threatening a), and forcing . White loses one point this way.
Let's play a difference game