With the Remote Control feature both users can see the same screen, keyboard and mouse input.

Please follow the steps below to establish a Remote Control session:

1) To initiate a Remote Control Session please start an RDP session with administrative rights and start the XP/VS admin tool.

Then right click on the session you want to Remote Control and choose Remote Control from the local menu.

In the next screen choose the key combination you want to use to end the Remote Control session.

2) After you have started the Remote Control session request the target user will see a confirmation dialogue in his RDP session to accept your Remote control request.

3)in following screenshot you can see the Remote Control session in action

4) To end the Remote Control session please press the key combination defined in the next screenshot .

IMPORTANT!

A) Remote Control can only be done from one RDP session to another RDP session or from one RDP session to the system console, but not from the system console to an RDP session.

This behaviour is exactly the same as with Microsoft Terminal Server.

B) Only a user with administrative rights can initiate a Remote Control session.

c) Remote Control is not longer supported with Windows 8 (all versions) nor Windows 2012
To answer the question as to "Why" and official response can be found here: 

"[...] The deprecation of shadowing in Windows Server 2012 RDSH was not a decision we easily made.  Based on the overall improvements moving to a new graphics pipeline architecture (see related blog posts at http://blogs.msdn.com/b/rds/) the current shadowing architecture no longer works.