The display is mechanically linked. Elongated cartilage spines radiate through the frill and attach to the hyoid, the skeletal support of the tongue and throat. Opening the mouth swings those rods outward and stretches the skin flap into a near-circular ruff, often flushed with orange or red at the base in the animals of the far north.
The display is used against predators and rivals. A threatened frilled lizard faces its aggressor, gapes, flares, hisses, and lashes its tail; if that fails it turns and runs on its hind legs to the nearest trunk, then climbs and keeps the trunk between itself and the threat. Bipedal running is escape, not habitual locomotion.
A point often got wrong: the frill has no venom, no spray, and no toxic function of any kind, despite a persistent film-derived association. Its size relative to body length is what makes it work, since a startled predator sees an animal that has apparently trebled in width. Whether the frill also serves thermoregulation or signalling in non-threatening contexts is debated.