One of the new features in Windows Server 2008 Terminal Services is Server Drain Mode. When a terminal server needs maintenance, the administrator may want to prevent users from logging onto a server which is about to be taken offline. In Windows Server 2003 there was a command-line tool to disable all remote connections; however, this prevented users with disconnected sessions from reconnecting, causing them to lose their work. To address this issue, in Windows Server 2008, the TS Server Drain Mode prevents new users from logging onto the server, while allowing currently logged on users to reconnect to their existing sessions. By waiting for existing users to save their work and log off, the administrator can take a terminal server down for maintenance without causing user data loss. Additionally, TS server drain mode is integrated with Session Broker Load Balancing, allowing one of the servers in a load balanced farm to be safely taken offline without any perceived change to end user experience or loss of service.
Source:→ Performance Team Blog