Snap the bottom of a soda can: it will dent inward and stay that way. Press again: it pops outward. That's bistability: a system with two stable states, like a light switch. An elastic arch is the simplest example. It can curve upward or downward, and to flip it, you need to apply force.
Scientists took an arch made of a material that changes shape on its own when exposed to light. They learned to control exactly how the arch snaps: which way it goes, how abruptly, and at what force. The secret is that the arch's shape can be broken down into simple waves, like sound into notes. If you change the shape so it matches the first wave, the arch snaps up or down. If it doesn't match, the arch transitions smoothly to a new shape, without a snap.
This discovery could lead to tiny switches for microelectromechanical systems (MEMS), robots, and smart materials. For example, you could make an arch that triggers with the slightest stimulus, or one that snaps only in one direction. It all depends on the shape you give it.
The arch's snap is like popping a bubble in bubble wrap: you need to press hard enough, and it suddenly transitions to a new state.
In science fiction, such switches could form the basis of adaptive armor that changes shape upon impact.