If the index service and query service are running on separate servers, the Search service has to copy the content index from the index service server to the query service server. The name for this process is propagation. During propagation, indexed content is not immediately available for Search; there is a delay until the content has been propagated.
Office Server Search introduces some changes that affect how propagation occurs, to reduce problems associated with propagation delay.
Following are the changes you can expect:
Single index for all content
Continuous propagation now available
With continuous propagation, only the changes to the index are copied, instead of the complete index, each time propagation occurs.
Propagation Object Model
The Enterprise Search in Microsoft Office SharePoint Server 2007 Administration object model provides access to monitor the status of the propagation system.