Take a strip of spring steel, wind it tightly into a coil and it will try to unwind. Harness the unwinding force, by connecting the spring to rotating gears, and you have a "spring engine''. The alternative name is "clockwork motor'' because this is how many clocks were driven before electricity came along.
However, a radio needs electricity, and so cannot be driven directly by clockwork. Trevor Baylis used a spring engine to drive a tiny electrical generator, the whole power pack being built into an ordinary sized portable radio. This is why the Baylis radio needs no batteries nor any outside power source, and is what makes it attractive in places like rural Ghana, where a few hours worth of batteries costs pound;10, an amount which is beyond most families.
A development project or agency can put Freeplay radios into a village for about pound;30 each.