Encapsulation occurs when a movement organization develops an ideology or structure that interferes with efforts to recruit members or raise demands. …members may develop such strong cohesion among themselves that outsiders become unwelcome. In prolonged interaction, a group may develop an ideology that is internally coherent but virtually unintelligible to recruits and outsiders who do not share all of the members’ assumptions. Such groups are not uncommon in movements; they constitute the fringe of organizations that appears strange to outsiders. An encapsulated organization may find it easy to maintain its dedicated core of members, whose identities are linked to the group and who may have few outside contacts, but such groups have little chance of growing or increasing their influence. Most strikingly, they may lose interest in such things, contenting themselves with maintaining their encapsulated existence.
[Frederick D. Miller. The End of SDS and the Emergence of the Weatherman: Demise through Success. Jo Freeman and Victoria Johnson, editors. Waves of Protest: Social Movements Since the Sixties (Rowman and Littlefield, 1999)]
This quote comes from a rather good article on a blog called “Devoke the Apocalypse,” which is well worth a look.