Not my words but the term for what you've just "discovered"
Keying (graphics) - Wikipedia, the free encyclopedia
My comments may seem pointless to you since you know what your video is about. For the rest of the world who look, all we see is someone doing something in an editing program with no explanation, guide to what you're doing or commentary of any sort. If it's supposed to be an introŽduction to lumakey, then you should try to inform or instruct.
Also, I used the term "copyright-free music" which you felt the need to point out was "free sound loops not copyright protected".