A scientific finding called “Flash Faced Distortion Effect” is a type of illusion that when when two faces flash, facial features appear distorted by your brain. In this example of the flashing female faces it makes the women look like zombies. The amount of the distortion depends on how much the features deviates from the rest in the set of flashing faces. As you watch the video, and I suggest doing it more than once (try it once with one eye closed), you’ll notice that someone with slightly larger eyes is perceived to have much much bigger eyes. If you slowly go through the video you’ll notice they aren’t zombies, they’re normal looking women.
If you want to read the paper on the illusion, it’s called “Flashed face distortion effect: Grotesque faces from relative spaces”.