How can I achieve this look?
 

YouTube - Kanye West - Homecoming

I am talking about the masking technique. I know how they did it with Martin over the white/gray backdrop, that is simple enough, but how did they add another image behind it?

This is simple:

http://img22.imageshack.us/img22/7030/picture5bt.png

Here is my test shot of it:

http://img69.imageshack.us/img69/8666/picture8pa.png

But these two shots are confusing:

http://img535.imageshack.us/img535/2996/picture7ia.png

http://img339.imageshack.us/img339/2498/picture6mj.png

Looks like they made the mask an alpha layer, made it transparent, and had video on an underlying track.

Try this.... Chroma key off a skin tone instead of blue or green. You should get the same effect as the piano shot as long as the person's skin tone is relatively even.

With the hands playing piano, it seems like the hands were keyed out, this could have either been done by keying out the skin tone, or the actually painted his hands green and then keyed in post.