Copypaste de un artículo de Timezone:
"Both the 21J Seiko and the 17J Heuer can be wound manually - both unidirectional. This is an advancement the 6138 holds over the 6139. Manual winding, though strictly not required, facilitates starting the watch from a stop after a few days of non-wear. This avoids the ungainly slow-shaking routine prior to setting, and also allows for a more constant state of wind (supposedly to facilitate consistent accuracy)."