As a retired emperor he was able to use power equal to an emperor under the provisions of the ritsuryo system, and had the advantage of being the real father of an emperor, so political power shifted from the regents and chancellors to the retired emperors.
律令の規定上、上皇は天皇と同等の権力行使が可能であり、天皇の実父であるという強みも得て、政治権力は摂政・関白から上皇へシフトしていった。