Within any job, there is what your role is on paper, and what you actually do over the course of a work day. Often times what you actually do is much more than “expected”. Work to rule means that you stick explicitly to what your role is, and the way the company expects you to perform your duties. Or rather, what they expect on paper.
An example: Unionized employees will often “work to rule” as one of the first steps aimed at putting pressure on an employer to negotiate. It’s an entirely legal thing to do, and serves to exemplify the disconnect between on paper job expectations (what you are paid for), and real expectations.
Thus, working as per your contract/job description with minimal deviation, will minimize how much you are exploited by ensuring you only do the work out are paid for.