It is easy to understand. Image has to be on the ratina for ~1/8th of a seconds or more. If we move our eye ball to follow the dots, image cannot be persistently made on the ratina and we cannot see the green dots( they are expsoed for very short time). But if we keep our eye constant at a spot (it can be anywhere, not necesarrily at the center), then the real green dots can seen. Good one though!