It's important for students to regularly practice topics they have completed in the past, to keep their memory fresh. They can do this by using the 'Practice' button on Student Topics.
Doing this will launch an adaptive problem set for your students, starting at the level of mastery they had previously achieved from any source (represented by the green bar). Even if the topic has already been mastered, students can continue practicing indefinitely.
The adaptive algorithm will continue working in this case - easier questions will be given if the students start getting questions wrong, even though the bar does not move.