[asherkin]

It clears out SM's internal cache of admins and fires OnRebuildAdminCache, which is implemented in the admin source plugins to reload their data into the cache (if they're not dynamic, like admin-sql-threaded). admin-flatfile will reload admin_simple.ini and admin.cfg, admin-sql-prefetch will load admins from the database, etc. admin-flatfile shouldn't cause any noticeable impact unless your disk is dying, admin-sql-prefetch almost certainly will as it has to use blocking SQL queries, obviously 3rd party plugins could be doing whatever.