After , since White a is not possible (Black will capture at b), Black lives as a twisted four. Now c and d are miai.
Black dies by bulky five.
Playing at looks good, but will capture the marked stone. However, has to be careful not to play at a or b.
is the vital point here.
is probably the weakest move among the failure diagrams.
If white b, then black c dies. So after white b, a and d become miai for black.