As part of the complete re-architecture of the Remote Desktop Services Roles in Windows Server 2012, Microsoft moved the cheese in a serious way.
While there is much that is great about the new design, and we applaud their decision to more holistically approach all of the disparate remote connectivity and VDI options provided by the OS, the UI is an incomplete, slow and confusing mess.
One of the biggest problems was that they severely hampered the built-in Administrative RDP connections that have existed since Windows 2003, and got rid of all of the old TS/RDP Management Tools. Even more painfully, they turfed everyone's old friend RDP Session Shadowing. Thankfully this has made a return in Windows Server 2012 R2, but the management and UI haven't improved much.
As a result of all of this, it became difficult to do the simplest of tasks. One of the most common that many admins used the Remote Desktop Services Manager utility for was connecting to another session, or ending a hung or disconnected session. Alas, no RDS Manager for you dear 2012 User. Too simple!
What to do? Task Manager. Seriously.
While this functionality has existed for quite some time, it is (by my quick survey of admins) unknown and rarely used. It is, however, surprisingly functional.
Simply bring up Task Manager, click More Details to switch to the "Actually Useful" mode, and then click the Users Tab. All current sessions will be displayed, along with a list of their processes. Right click on any session to connect to or end the session.
Simple, but non-obvious.