Katherine Rawson and John Dunlosky’s 2011 paper established the finding: spacing alone works. Retrieval practice alone works. Combined, they produce effects greater than the sum of the two — superadditive, not additive.
The intuition is that spacing solves one problem (the forgetting curve) while retrieval solves another (shallow encoding). Each addresses a different bottleneck. Together they address both, and the bottlenecks interact multiplicatively.
Meta-analyses consistently find successive relearning at or near the top of the evidence-based study-strategy rankings, across domains and learner levels. Fluera’s spaced repetition scheduler is not flashcard-based retrieval and not calendar-based spacing — it is the combination, on the knowledge structure you already built.