There are certain people with braces who have “rubber bands” either forming an X across the maxilla and the mandible, or just a single rubber band pulling both the maxilla and mandible together. Those rubber bands have a purpose, and that is to help people who have “over bites” or “under bites” correct their jaw alignment. Although some have other reasons why they attach rubber bands to their braces (i.e. to pull a hidden tooth inside the gum), over bites and under bites are the most common concerns addressed by these elastics. What these orthodontic rubber bands do is they pull both the maxilla and mandible towards the other direction.

Photo taken from http://library.thinkquest.org