A number of ways, for instance from what I read in the New York Times article a couple days ago. First is direct political activity -- anybody in city governance who crosses the union loses their seat in the next election.

Second, the union contract can effectively dictate how laws are enforced on the police, e.g., the what the disciplinary mechanism is like, so the laws become un-enforceable.

Third is through labor action such as work-to-rule.