When a red square flashes on the left they shift to the right.
A subsequent red square on the left should be ignored, since the subject is now paying attention to the right side.
When a red square appears on the right, the subject shifts attention back to the left, and so on. I've slowed these stimuli down so that you can see them better. Imagine them speeding by at eighteen frames a second and you'll have an idea how difficult this task is.