A PMHNP is explaining to a resident why aripiprazole, a partial dopamine D2 agonist, can function as either a functional agonist or a functional antagonist depending on the prevailing dopaminergic tone. In a patient with schizophrenia, aripiprazole reduces positive symptoms (suggesting D2 antagonism in the mesolimbic pathway) while causing less parkinsonism than full D2 antagonists (suggesting some D2 agonism in the nigrostriatal pathway). The resident asks how a single drug can have opposing effects. Which of the following best explains partial agonism?