Billy:

You're an expert in this field. Can you explain the reason behind?

Easy.
The cross is to help your eyes keep steady.
The discs disappear due to the adaptation effect of the receptor cell in retina. That's also the same that u can't see your blood vessels on your retina UNLESS the light comes from an oblique angle (e.g. from side way).

The remaining green disc is the complementary afterimage of the disappeared purple disc. That means the green disc appears right after the purple one is gone.
Complementary means that, if the disc is in yellow colour then u'll see the blue disc moving around. If the disc is in red colour then u'll see the green disc moving around. Just like after seeing the flash from the camera, u'll get a black afterimage.