Flashcards is a discontinued feature of the Duolingo site, which was used to quickly practice words previously learned on Duolingo. Users would grade themselves as they are shown words in the language being learned and their translations. This replaced the "Practice weakest words" button, which had functionality identical to the strengthen skills button on the home page. Duolingo now offers the use of the Tinycards app instead for revision of vocabulary with this method.
Practicing with flashcards strengthened words, but the "decay rate on words that you mark wrong, however, is much higher than for the words that you mark right. This means the words you didn't know will lose their strength faster over time than the words that you did know, even if they are initially bumped up to the same strength."
Flashcards were different from normal practice: