In rapid maxillary expansion appliance, when a rapid heavy force is applied to the teeth, there would not be enough time for tooth movement to take place; hence, the force is directly transmitted to the sutures.  The appliance compresses the periodontal ligament thereby bending the alveolar process, tipping the anchor teeth, and gradually opening the midpalatal suture.  The sutures open with minimal movement of the teeth. There is substantial increase in the maxillary width with fixed appliance than removable appliance. ​Textbook of ORTHODONTICS Sridhar Premkumar