A 9-year-old boy with ADHD, combined presentation, has been taking methylphenidate extended-release 54 mg daily for 10 weeks with only marginal symptom improvement. His teacher reports continued difficulty sustaining attention, frequent off-task behavior, and incomplete assignments. Side effects have been minimal, limited to mild appetite suppression at lunch. His parents ask whether trying a different stimulant class would be reasonable. The PMHNP explains the pharmacologic rationale for switching to a mixed amphetamine salts preparation. Which of the following best describes the mechanistic difference that supports this clinical decision?