Why does a spaced-repetition flashcard app work — in terms of the spacing effect and the testing (retrieval) effect?

It interrupts the forgetting curve just before you'd forget (spacing) and forces you to recall rather than recognize (effortful retrieval) — the two strongest results in memory science.

This is the payoff of the whole mission — and it's why this very app works. A well-designed spaced-repetition system does two things at once:

1. Times reviews near the moment of forgetting — retrieval strength has dropped, so the review is effortful and banks maximum storage strength (the spacing effect + Bjork's principle).
2. Demands retrieval, not re-reading — flashcards and duels make you produce the answer, which is the effortful act that drives durable learning, instead of the hollow fluency of review.

The app isn't a gimmick layered on studying; it's engineered directly around the forgetting curve and storage/retrieval strength.