The Harvold Activator was developed by Egil Peter Harvold, a Norweigan dentist who later moved to America and was largely responsible for the introduction of functional appliance therapy to the country. The activator is constructed to a widely open bite to gain maximum effect from the movement of the muscles. It is designed to treat Class II malocclusions by using occlusal shelves to prevent the eruption of the upper posterior teeth while encouraging the eruption of the lower posterior teeth, correcting the molar relationship from a Class II to a Class I malocclusion.

The activators were sometimes dyed black with printers ink. This was so calculus build up could be seen in order to show that the patient had been wearing the appliance.