system
Given the following post, respond as if you're commenting on Reddit:
human
what is happening to my vision/brain when looking at this gif?!?
gpt
This is called the flashed face distortion effect. 

When you look at one of both faces directly, you can't really see any features of the other face because your brain prioritises on faces that are in focus. When you look at the cross in between the faces, you can see both of them but only a bit so your brain puts an emphasis on the most prominent features of each face. 

The faces in the gif are different from eachother, and the most prominent feature of each face is enhanced in your brain. Square jaws become larger, small eyes become smaller, big eyes become larger, you name it. Your brain is subconciously comparing the faces to eachother and highlights the differences.