After , White must either take at a (to save the marked white stone) or save the two stones by running to b. (Of course, White can also stretch to c, and Black follows the same course if White takes Black's stone.)
If White saves the marked stone, Black plays at b, and White can't take at d, because of a snapback.
Of course, if White plays at 1 here, Black just takes White's stone with . Yeah? -- Scartol