The initial rotation plan divided North America into three zones (Eastern, Central, and Western) with the site of Worldcon rotating among them. Each year, bids would only be in order for one zone. Bids from the Rest of the World, however, were always eligible, and, if successful, was inserted into the rotation, pushing the cycle ahead one year. (I.e, if a Central zone site was eligible in year N and a Western zone site in year N+1, and a foreign bid won for year N, Central zone sites would now be eligible in year N+1 and Western zone sites in year N+2 and so on.)
See a discussion from the mid-60s in the Discon 1 Guide: WSFS.
This scheme worked well, except that as the length of bids and the advance time needed to secure facilities grew longer, it became impractical to shift bids by a year if a foreign bid won, and the rotation scheme was changed so that a winning foreign bid preempted the turn of the zone it replaced. (So if a Central zone site was eligible in year N and a Western zone site in year N+1, and a foreign bid won for year N, there would be no Central zone sites this cycle of the rotation and Western zone sites would continue to be eligible in year N+1 and so on.)
With very minor tweaking, this system continued in place until 1999-2002 when it was replaced by the No Zone system. Zones were eliminated so that any site was eligible any year except that no site located within 500 miles of the site conducting the election is eligible. This change was made because, due to increasing competition with mundane conventions, it had become harder and harder to get suitable Worldcon sites and it was judged necessary to provide bidders with more flexibility.
|from Fancyclopedia 2 ca. 1959|
|In 1950, after a series of conventions had taken place in the Eastern half of the continent, a cry went up for a Pacificoast convention in the name of fairness. It was also thought that some formula for describing eligible convention sites should be adopted, to prevent a preponderance of local fans voting the Worldcon for their region year after year. The idea finally adopted, at the PhilCon II, provided for conventions in the East, Middle States, and West ("orderly progression westward") successively, with conventions outside the US not counted as stages in the rotation. Acceptance was general and the idea was one of the customs formalized by WSFS.
from Fancyclopedia 2 Supplement ca. 1960:
The Rotation Plan was not "adopted" at the 1953 Philcon II, but only advanced (By Doc Barrett, Lou Tabakow, Don Ford and other Cincy fans). The idea was filibustered to death there, but was adopted at the 1954 SFCon. And of course the actual procedure just reverse the quoted slogan, being actually an "orderly progression Eastward!".