is sente because if is played somewhere else Black can play at a and White won't be able to protect against the capture at b because of lack-of-libs.
zinger: Or, more simply, black b would be double atari.
A common technique in semeai and such.
Now thanks to the throwin White can't play at a (If White does, black simply plays at b and captures more stones) because of another lack-of-libs. Black captures the 3 stones if so and lives.
unkx80: Yes, this is the solution. It is the failure diagrams that needs a lot more talk.