[Warning: this is a rather subtle topic]
What to observe
So what do you see on the right? Clearly a red and a blue disk. Do you see movement from the left to the right? If so: at what time, or rather at what place in the trajectory does the colour change?
Actually, the red disk on the left disappears, and the blue disk on the right reappears after a brief interval (default: 80 ms).
What to do
You can change 3 timing parameters, the distance of the disks (with the slider) and the colours.
As I wrote above: it's a subtle effect, some people just see two disks appear after another. If you do have a percept of motion, two things are at work here:
(1) The “Phi” phenomenon. That refers to the observation that we can perceive continuous movement although only a sequence of static images is presented, this is also known as “apparent motion”. Max Wertheimer (1912) called it “phi” because he studied circular motion where the angle phi parameterised the step size. The phi phenomenon is the basis of all movie and TV technology.
(2) “Colour Phi”. Now comes the deep thing: Those observers, who do see motion in this situation, may also observe the colour change “along the way”, e.g. at midpoint. This was first described by Kolers and Michael von Grünau (1976, I knew the latter well, he sadly passed away much too early). That we can see an object turn blue when it doesn't exist yet, is amazing and made much use of by the philosopher Daniel Dennet in his “Consciousness explained” (1991). See Selmer Bringsjorde for a counter argument.
I myself am quite unsure what I see here.
Johnson G (1991) New York Times Book Review of “Consciousness Explained”
Kolers PA, von Grünau M (1976) Shape and Color in Apparent Motion. Vision Res 16: 329–335
Wertheimer M (1912) Experimentelle Studien über das Sehen von Bewegung. Zeitschrift für Psychologie 61:161–265 [PDF]