I spent some years working as a front-line supervisor in a factory that was unionized. It was obvious, to me, that one of the best tools available to me was a good knowledge of the union contract, especially the sections that pertained to assigning people to tasks, handing out overtime and disciplining people.
It was also astonishing how many supervisors and higher-level managers didn't have that knowledge, including people in HR.
From what I'm reading here, things aren't much different in managing police departments.