fivebells?: ladders , leaving plenty of liberties for that black group. Then leaves plenty of liberties for the group in the corner. This means that there is no contest in a capturing race for the stones, and there is no eye space for them. There doesn't seem to be any effective response for white in between these two black moves.
This seems too simple, though. It's likely that I have missed the point of this exercise, or I've missed some tactical difficulty with the plan I've outlined.
symplicity: When white runs, the marked black stones are in atari. What does black do then (or what does black do instead)?
fivebells?: Ah, thanks for pointing that out.