When presented with Class III occlusion or an underbite, the lower teeth and jaw will be ahead of the upper teeth, opposite of ideal occlusion. In growing patients, this is treated by bonding an appliance on the upper arch that contains hooks along the sides.
A facemask portion will also be worn externally, placed on your forehead and chin. Elastics will be stretched from the hooks on the appliance to the arms on the facemask, encouraging the upper jaw to grow forward. This will slowly pull the upper teeth and jaw ahead of the lower jaw, into a more ideal relationship.