The biggest problem I had with Duolingo was knowing what to do next. I much prefer a simple "next lesson" button, which both introduces new material and reviews stuff you've covered previously. But with Duolingo there's a set of available lessons you can choose, and you can either work on going down the list and opening up new lessons, or re-doing ones you've already done and leveling up those particular ones. Then t…
It takes you into a lesson that it thinks you should review next. Duolingo is meant to implement spaced repetition in that it keeps track of all the times you have seen every word, and it tries to give you lessons that are "due" for repetition according to its algorithm. In the web interface they show you how "strong" each word is in their opinion, but I found that their data on when you last saw a word was buggy.
Anyway, the barbell is useful to keep you up to speed by revisiting stuff once you have finished all the lessons once.