I forgot to turn my video recorder off and accidentally recorded my screen when I watched some series on some website. When I re-watched the video a few weeks later, I noticed that the part where the series was played (on full-screen) was a totally plain black screen. I found it weird so I thought it was a bug or something but I discovered it's not. It was apparently some kind of protection to prevent video pirating and the idea just wow-ed me.
I'm a fresh CS student who's taking a lot of courses in security but I have no idea how does that idea work especially because I could see the video but the video recorder couldn't. Can someone explain how that works?