When bone breaks, it naturally regenerates to fix the fracture. Lengthening works by separating the bone and distracting (pulling apart) the bone segments very slowly so that new bone continues to form in the gap. As the bone segments are slowly distracted, the bone regenerates, resulting in increased length.
External fixators are devices that attach to the bone through wires and pins. The external fixator looks and acts like a scaffolding, supporting the separated bone. The fixator mechanically lengthens the limb by pulling the segments of bone apart.