You're talking about SRS (Spaced Repetition - http://en.wikipedia.org/wiki/Spaced_repetition_software). We don't technically do SRS (yet -- although we're thinking about it).

But our "Learn" mode does keep track of what you know and what you don't know, and keeps asking you the terms you have trouble with until you know them all.

Hit me up by email if you have any more feedback for me - would love to keep improving the product.