Review
Every concept you answer wrong comes back in two days, then seven, then twenty-one. Answer it right and it moves out to the next interval. Answer it wrong and it starts again.
The gap is the point. Revising something while it is still fresh feels productive and does almost nothing; being asked again just as it starts to fade is what makes it stay.
The schedule lives in this browser. Clearing site data clears it, and it does not follow you to another device.