A snap band, also known as a slap bracelet or slap wrap, is a flexible, springy strip of metal sealed within a decorative fabric, silicone, or plastic cover. Tapping it against a wrist, ankle, or other object causes it to snap instantly into a curved band.
How they work
The mechanism of a snap band is an example of a bistable spring. This refers to an object that has two stable states, with the band being either flat or coiled. A small amount of force, like a gentle slap, is enough to trigger the band to snap from its flat state into a coil. The inside of the band is a piece of bistable spring steel, often from a repurposed steel measuring tape.