Let’s consider rule utilitarianism:
Ultimately, we strive to maximize human welfare. When we ask ourselves, how we should act, we imagine that our actions become general rules. For example, if I am a poor orphan, does the rule Steal food from rich men“ lead to the greatest overall happiness? If yes, so it. If it does not, don’t.
Now, let’s imagine an exception. In some exception, stealing from rich men is fine, because it leads to the greatest overall happiness. An act utilitarian would break the rule. But a rule utilitarian would make an exception and change the rule.
Thus, the motion is the same: whenever an act utilitarian breaks a rule, the rule utilitarian makes an exception to the rule.