Paira schedules every word separately. Answer correctly and the word moves up a mastery level and waits longer before returning. Answer wrongly and it drops a level and comes back immediately. Everything else is detail, and this post is a map to where that detail is written up.
The intervals
Six mastery levels, from 0 to 5, each with a waiting period attached. Climbing from zero, the gaps you actually get are 1 day, 2 days, 4 days, 7 days and then 14 days, which is the ceiling.
The full interval ladder, and what a wrong answer does to it
Which words a session picks
A word is new, due or weak. Due means its date has passed; weak means you have got it wrong before and have not yet driven it to full mastery, which lets it arrive ahead of schedule. A session interleaves roughly half review, a third new, and the rest a refresh of things you are not struggling with.
The review queue is built across every set you study rather than one set at a time, because your memory does not file words by which list you typed them into.
What makes a word weak, and when that flag clears
Why that split decides how many new words you meet
Answering twice in ten minutes does not count
A correct answer given within ten minutes of the last one that counted is recorded, but it does not advance mastery and does not push the review date out. The answer is still in your working memory, so it proves you can repeat something you just read rather than that you will know it on Thursday.
It runs underneath every mode
The schedule is not a separate mode you have to remember to open. Timed Match, Smart Learn, flashcards, quiz, typing, listening and the rest all report into the same per-word record, so playing is what advances it. How much a correct answer counts does vary by mode — a True or False answer is worth half of a typed one.