This tutorial explains the details on AEM re-fetching flush agent and how to use to flush the content through re-fetching flush agent.
Re-Fetching Flush Agent
With regular flush agent, the agent flush the cache and the flushed content would only be retrieved from the publisher again after first time requested. This will create issue if the multiple parallel requests hit the dispatcher after flush as the new page is not yet cached in the dispatcher.
The re-fetching flush agent in AEM make sure the pages are requested and cached on the dispatcher immediately after replication.
Using a re-fetching flush agent allows to pre-populate the cache as it instructs the dispatcher to retrieve the pages from publish instance immediately after the flush occurs. Re-fetching flush agent works same as normal flush agent in dispatcher, additionally dispatcher sends a GET request for the content paths in the flush requests to fetch the content and update the cache.