This feature is available from BI 4.1 SP06 onwards.
Prior to SP06, when an open Document, CMC, BILaunchpad is accessed and if user closes the browser, user session remains active and is dropped only after 20 minutes (default http session timeout value).
As user session remains alive the corresponding license is also not released. Especially for Open Document there is no way for the user to log off explicitly. This Impacts the number of concurrent licenses.
From BI 4.1 SP06 onwards, if browser with open document link with active session is closed then the current user session is logged off and therefore the licenses are immediately reclaimed.
Similarly Licenses are reclaimed on browser close of BILaunchpad and Central Management Console also.