Zealous Representation: What Does It Mean?

There was an interesting article in the Times today about a Manhattan assistant district attorney who intentionally lost a case where he did not believe in the conviction.  According to the Times article, the responsible court agency (which I presume is the First Department Disciplinary Committee) chose not to purse a case against Mr. Bibb.  The case raised the question of what zealous representation means when one is a district attorney.