This rule can be seen as a relational network among events. As you follow the rule, you can compare the relations among the events as they happen to the relations specified by the rule. Basically, the person following a rule can detect that the rule is being followed (or not) because what is being done corresponds (or not) to the relational network specified by the rule.
If you feed my pig while I’m on vacation for the next week, I’ll give you $50 when I get back.