Spelling Tutor repeats words you get wrong on Day 2, 3, 5, 8, 13 and 21. Each time you get it wrong it goes back to the beginning. So each word is presented until you have got it right 7 times in a row over 21 days.

Research shows spaced repetition works because people more easily remember things when they are studied a few times spaced over a long time span rather than repeatedly studied in a short span of time.

This is why learning words for an end of week spelling test often doesn't improve spelling. The words are soon forgotten. Whereas with spaced repetition the words are transferred from short term to long term memory.