I really want to help people understanding the basis on which they can go further. If students are behind classes, I would trace back to the point where students understand most and from there, we can step by step go to the current lessons. In case students are up-to-date with classes, I will go deeper into the lessons to point out why what is what, then we can try to go ahead of current lessons (so that when student comes to class, it will be much easier to understand what the teacher says).