Alternate residence duty (sankin kotai) demanded that a daimyo or lord alternate residence at the Tokugawa castle at Edo and his own castle. When the daimyo was at home, he had to leave his family in Edo. Thus, it was a hostage system which left either the daimyo or his family always subject to the overlord’s control. The system also imposed a heavy financial burden; the cost of maintaining two homes plus traveling to and from Edo. These expenses often amounted to 25% of a lord’s income.
More extensive information can be found at the large entry on Sankin Kotai and the Hostage System.