what is the algorithm behind cardlearner

I have read the instructions，but didn't make sure what algorithm it is based on.
It is just mentioned that "Repeat hard cards only". But how to repeat?
most of flashcards apps are based on spaced repitition system, like supermemo,anki.
some others are based on leitner system.
Without good algorithm, there will be no efficient learning program. so,what kind of algorithm is this app based on?

Hello!
I don't know how to name it, but concept is:
When you had learned dictionary from the beginning to the end, CardLearner analyzes number of errors for every card and mark it as a "hard card", if this card was more difficult to study then most of others.

But how to repeat?

Just open once thoroughly learned dictionary and press top right button. It separates hard cards.