Rabbinic conventions can--following specific guidelines--amend Jewish law.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
Rabbinical synods were assemblies or conferences of Rabbinic leaders at which rulings were given governing the social life of Jews under their jurisdiction. In the Middle Ages the synod was known as the asifah ('assembly').
The Rabbinical synods differed in two respects from the synods of the Church. First, the Christian synods were convened largely for the purpose of defining complicated issues of dogma, whereas the Rabbinical synods were chiefly concerned with practical legislation.
Secondly, the Christian synods enjoyed an international authority, whereas the Rabbinical synods in the Middle Ages were confined to particular communities or districts. Once the period of the Geonim had come to an end there was no central authority for Jews.
What are Takkanot?
The main activity of the Rabbinical synods was to establish takkanot ('enactments'). A takkanah ('putting right') consists of new legislation to cover situations for which the standard laws are inadequate or on which they are silent. The principle behind the takkanah is that locally accepted authorities have power, granted to them by the community itself, just as members of Parliament act on behalf of the country.
A synod had the power to issue new financial and social regulations, at first binding only on those under the particular Rabbinic jurisdiction but often finding their way into the Codes of law, when they thus became binding on Jews outside the original communities in which the takkanot were promulgated.
For instance, the famous ban (herem) on polygamy, attributed to the Synod of
Rabbenu Gershom of Mayence, became binding eventually not only on Jews living in Germany but on all Ashkenazi Jews. This extension of the ban did not apply however, to Sephardi Jews, who, nevertheless, introduced certain restrictions of their own against a man taking more than one wife, for instance by encouraging the drawing up of a ketubah (marriage settlement) in which it was stated that the consent of the first wife is required before her husband can take another wife.
Such Sephardi regulations were also limited in that they applied only to the particular community in which they were promulgated. Similarly, the ban against the study of science and philosophy by youths under the age of 20, promulgated by the Synod of Barcelona on 26 July 1305, under the leadership of Solomon Ibn Adret, was binding only on the Jews of Barcelona, although attempts were made to extend the ban to other communities.
The idea behind it all is that the synod enjoyed its powers by the consensus of the community, so that any legislation that emerged was binding by the community's acceptance, not in any way because of the dictate of the Rabbis themselves.