How can a wound spring move a toy?

A small toy can roll across a table after the hand that wound it has let go. Its energy was put in earlier. Turning the winding key changes the shape of a spring, storing energy as the spring is bent or twisted away from its relaxed shape.

A latch holds the wound mechanism in place. The spring can still push on its connections, but those parts cannot yet turn freely. Releasing the latch removes that restraint; it does not create the energy that starts the toy.

As the spring relaxes, it turns a connected shaft. Gears can pass that turning movement to the wheels and change the rate at which they rotate. The spring, shaft and wheel connection form a route for energy to travel from stored spring energy into movement.

The toy eventually slows because the spring has less stored energy left to release, while rubbing parts and moving wheels transfer energy into warmth and sound. A relaxed spring cannot keep driving the toy. The short journey is a visible result of work done earlier during winding.