Stare at the black + in the center, the moving dot turns to green.
Now, concentrate on the black + in the center of the picture. After a short period of time, all the pink dots will slowly disappear, and you will only see a green dot rotating.
the luminance of the background and and the pink dots is the same .. thast why it merges ... the green color is the opposite color of pink thats why it apperas ... it this was a still picture .. you'd only see a grey square