The daily exercise mode will let you set a goal as number of sentence you want to practice per day (5 - 10 - 15) and will remind you (if you want) to come back and practice it.
Useful to have at least a 5 minutes workout everyday ;) (Which is a minimum you should go for more, I believe in you! )
The algorithm behind will take a random sentences between all of the sentences in the app and remember that you already seen it.
After seen it more than 5 times the same sentences, it will remove it from the list of sentence that the app can propose you, because we assume that you know it now!