In the second video it's done by locking down the camera. The actor moves his arm without the coat on and then does a second take with the coat. The key is that his location, eyeline, and arm movement must be as exact as possible.
Finally in post he composites the shots together with using a simple mask.
For youtube videos, if you go to keepvid.com, you can download high res versions of the video, drag them into AE and watch them frame by frame---I find it very handy when trying to break down shots.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.