Spaced repetition exploits the "spacing effect" — one of the most robust findings in cognitive psychology. Learning distributed over multiple sessions with increasing intervals produces far more durable memory than the same amount of time in massed practice ("cramming").
Ebbinghaus's forgetting curve (1885) established that memory decays exponentially after learning. Spaced repetition schedules review just before forgetting would occur, forcing retrieval and strengthening the memory trace each time.
Modern implementations like SM-2 (used by Anki and MindFrame's challenge scheduling) adaptively set review intervals based on your recall performance. Challenges you find difficult appear more frequently; challenges you master are spaced further apart.
The practice effect for metacognitive skills specifically requires spaced repetition — a single exposure to calibration feedback does not produce lasting change. MindFrame's SM-2 scheduling is designed around this requirement.