User blog comment:RaijinRising/Every Time Forever Fall's Music is in RWBY/@comment-26018514-20190823173412/@comment-4010415-20190823182029

You may have to use the trick that reaction Youtubers who don't have a facecam use - put the video in a smaller rectangle with an animated background behind it. This fools Youtube into not recognizing the copyrighted content... visually, at least.

But yeah, having a moving background around the RWBY footage is what keeps the reaction videos from being taken down. Reactors who don't have a webcam to serve as the moving background have to put an animated background of some kind.