Here's how it works:

1. Winding the Clock: When you wind a mechanical clock, you are storing energy in a spring. This spring is typically a coiled metal spring.

2. Spring Uncoiling: As the clock runs, the spring unwinds, releasing the stored energy.

3. Power Transmission: The spring's unwinding motion is transferred through a series of gears and levers.

4. Movement of Hands: This energy ultimately drives the movement of the clock hands.

In short, the force of the spring unwinding itself powers the entire mechanism of a mechanical clock.