If Black plays 2 like this, the situation becomes ko (difficult to win for Black, but still ko).
--Herman
Playing at 1 removes any chance of getting into ko. Black cannot play a, since White has one liberty more. If Black plays b, White plays c and vice versa.
--Herman
unkx80: Herman has given the correct answer. In this game I was Black. If White had played 1 as in the correct answer, I would have lost the game. But I won. =)