Transferred from solution page:
D7. With that move, you cut the white group and are exposed to being captured. If the White captures, play D6 again. If it doesn't D6 and game over.
in my opinion the above described play is not perfect. if black decides to play that way the bottom 4 stones are unconditionally alive. through: bd7, wc5, bc6, wa2
i didn't investigate futher but black might consider a play like: b5 where i imagine white completly dead.