CleverDeck uses the SM-2 algorithm. This is the same algorithm used by other popular spaced repetition systems such as Anki and SuperMemo. Several of the algorithm's parameters can be customized inside the app.
The SM-2 algorithm relies primarily on two variables to calculate new review intervals: the previous interval and the ease factor. The ease factor is simply a number that represents how easy CleverDeck thinks you can remember a card. The easier you have remembered a card in the past, the longer the next interval will be and vice versa.
The ease factor is initially the same for all cards. Remembering a card will increase the ease factor, while forgetting or struggling with a card will penalize the ease factor. This results in the SM-2 demanding more reviews for cards you have trouble remembering.
To avoid a situation in which reviews become irritatingly frequent, the ease factor can't fall below a reasonable minimum value
When you forget a card and swipe it left, it is immediately put into the back of the card stack. If a card stack is particularly large, the card will be placed at a consistent distance from the front to avoid having to wait too long to review it again. You should keep swiping a card left until you can finally recall it.
When a card is remembered after being forgotten (i.e. is swiped right after being swiped left previously), it is scheduled into the future by a constant value called the Learning Interval. The Learning Interval is 30 minutes by default, but can be configured in settings. In effect, the learning interval asks you to review cards again shortly after failing to remember them.
Following a Learning Interval, you'll either remember the card or continue to fail. When you fail, the process repeats: the card is first put in the back of the stack until you recall it and is then scheduled forward by the Learning Interval.
However, if you successfully recall a card following a Learning Interval, it graduates and is scheduled into the future by the Graduating Interval. The Graduating Inteval is 24 hours by default, but can be configured in settings.
Once a card has graduated, it enters the main scheduling loop where your confidence reports and the ease factor will determine subsequent intervals. The card will continue like this unless you eventually forget it, in which case it will exit the main loop and enter another Learning Interval. This isn't exactly equivalent to starting over - due to accumulated bonuses to the ease factor, the card will more quickly acheive longer intervals.
If you report that you can recall a new card the very first time you see it, the card is scheduled into the future by the (brilliantly named) Already Known Interval instead of the Graduating Interval. The Already Known Interval is three days by default, but can be configured in settings.
If you know a card the first time you see it, it obviously doesn't need as much attention as other cards you didn't know. The Already Known Interval is also accompanied by a generous bonus to the ease factor.
When in the main scheduling loop, a card's ease factor, previous interval, and your confidence report determine how long until you see it again. Since up-swipes are lower confidence, they will result in shorter intervals and a slight penalty to the ease factor.
CleverDeck also considers how much time has passed since the card triggered for review. For example, if you went for a long vacation and didn't open the app for 10 days, subsequent intervals will take into account the time you've been away. After all, if you were able to still remember a card 10 days after you were supposed to review it, then you obviously know it very well and the future intervals should reflect that.
By default, the SM-2 implementation includes a few constant values - such as the intial value of the ease factor. CleverDeck uses default values that reasonably fit the average learner.
However, some of us have better memories than others. If you feel like CleverDeck is too demanding and you frequently can't recall cards, decrease the Interval Multiplier to shorten all scheduled review intervals. Conversely, if you think you could wait longer between reviews, consider increasing the Interval Multiplier.
CleverDeck tracks a rolling average of your memory retention for each deck, viewable in a deck's settings. A good rule of thumb is to adjust the Interval Multiplier to keep this number around 85%. If retention is much less, you should probably review more often (smaller multiplier). On the other hand, if you are remembering everything easily, you could probably spend less time (larger multiplier) studying and achieve more efficient results.
You may find that you retain information in some decks easier than others. For example, memory recall can be especially difficult for languages that you've yet to gain an intuition for. In this case, you can optionally set individual Interval Multipliers for each deck. This multiplier overrules (i.e. doesn't combine with) the global one.
Once in the main scheduling loop, cards will remain there forever. If you keep successfully recalling a card, its intervals will eventually become so large that you will never realistically see it again.
While, technically speaking, the idea of "mastering" information is dubious, it can be a helpful and motivating way to display information. Each deck displays how many of its cards have been "mastered" - defined, by default, as being able to remember something after not seeing it for 21 days or longer. This value can be configured in settings.
To be clear, the Mastery Interval is purely a cosmetic setting. It doesn't actually affect how cards are scheduled.