In your video it looks like you have a main duct with seceral Diffusers conencting to it with short ductign between. The order of operations is as follows:
1. Click on Diffuser and choose Connect Into.
2. Pick the duct you want to connect too.
3. In Plan > Lock and Align the Diffuser to the Ref Plane.
4. Move the Ref Plane.
I'm not sure what expereience you have in the application but it you do not have any formal training it may be a good idea to get some. Also the Autodesk Wikihelp site for 2013 is full of great information on ducts and many other features in Revit. Go here for search results on Ducts: http://wikihelp.autodesk.com/search?source=help&pr
I have also places another video and file in my Dropbox. I did not do the above steps on one diffuser and duct. I suggest you practice in my file.
Well, The diffusers I use are Face Hosted, so I cannot place them in mid-air without a Reference plane to host to. I also don't have to "Align" and "Lock" the diffuser to the Plane.
I see you had to use the "Connect Into" to get it to work properly. Is that the only way? In previous versions, I had no problems with this. when you connect duct to a diffuser (manually), it should remain connected, even when you move the diffuser. Now, you have to tell the Revit you want it to remain connected, even after you physically connect it? What is the reason for the change?
I've been using Revit MEP since version 2008 (with formal training) , and Architectural since 2004 (my 10 yo. nephew could figure it out).
Please let me know why you have to use "Connect Into" to get the connection to stay.