The Iron Jaw is a traditional aerial circus act.
The act consists of a performer raised into the air by a cable, trapeze

Trapeze

A trapeze is a short horizontal bar hung by ropes or metal straps from a support. It is an aerial apparatus commonly found in circus performances...

 or other similar apparatus, supported only by a bit

Bit (horse)

A bit is a type of horse tack used in equestrian activities, usually made of metal or a synthetic material, and is placed in the mouth of a horse or other equid and assists a rider in communicating with the animal. It rests on the bars of the mouth in an interdental region where there are no teeth...

 clamped between her teeth. Depending on the apparatus the bit is attached to, the performer can alternately swing in circles around the ring below or spin wildly in place, controlling speed by alternately extending her arms and legs out or bringing them in close to her body to speed up (known as the conservation of angular momentum principle).

The act is frequently referred to as the Human Butterfly due to the removable wings which traditionally adorn the performer's costume.