The collective bargaining agreement (CBA) is where the power is, its a legally binding agreement between the Union and the Employer. The unions have lawyers that have crafted the language and there is a lot of back and forth until its agreed upon and then its binding for both parties.

When you get a union job, the position is represented by the union and the employer has agreed to the CBA. So as the union employee, if I feel that I am be asked to do something outside of my job description, I can request the union to audit the work which puts management under the microscope and well they don't usually like that so they will create a cold war and force policy adherence to the letter. In the end everyone is usually more clear about what is acceptable and what is not.

But the real power is that if its not in your job description, you can really say no and they cant do anything about it. The "other duties and responsibilities" should never be more then 8 hours in 2 weeks. If it does, it needs to be included as part of your job description. And the union has a death grip on that description with the CBA.

Laws obv vary by country, but in the US, a corporation is legally required to recognize and bargain with a union if over 30% of their employees choose to unionize.