Think of extra reviewing as additional practice sessions that you can do even if you haven't yet been prompted to review the words you've been learning in a course. Extra reviewing isn't considered the most efficient way to learn, because it means you are re-tested on words before you need to be.
The blue 'Review' button is seen next to a course that needs reviewing. The grey 'Review' button is the 'extra reviewing' function. It will simply test you on words you have very recently learnt.
Please note: You will earn less points during an 'extra reviewing' session than if you are prompted to review certain items.
Set a daily goal - 1500, 6000 or 20000 points
How do I quit / restart a course?