You are looking at a pattern of rectangles sitting in front of a multicolored background.
For most people, the pattern appears to expand or, if you press the “click to reverse direction” button, to contract. The perception of expansion or contraction (of “rainbow boom” or “rainbow bust”–an economic metaphor) occurs even though the image is completely still.
The key to the motion is the contrast between the colored edges on the rectangles and the gradient background. If the yellow edges are on the outside of each rectangle, the pattern appears to expand; if the yellow edges are on the inside, the pattern appears to contract.
The effect is reminiscent of the Pinna-Brelstaff illusion, which consists of diamonds with two white edges and two black edges, in ring formation:
Stare at the blue star, and move your head closer to and further from the screen. As you do so, the rings should appear to rotate.
Unlike motion in the Pinna-Brelstaff illusion, the perceived motion in Rainbow Boom/Rainbow Bust does not depend on the observer moving back and forth. I achieved this effect by placing the rectangular pattern on a gradient background. In this way, even though the rectangles are the same throughout the display, the contrast between the rectangles and the background changes from the center outward. It is the contrast that seems to be driving the perception of expansion and contraction.
What causes the motion? One possibility is that small eye movements lead to changes in the contrast response; the visual system creates motion from the differences in contrast across edges. Another possibility, put forward by Ben Backus and Ipek Oruc, is that the brain’s motion detectors fail to compensate for the dynamics of local adaptation. The Backus-Oruc theory was created to account for the perception of motion in a different static illusion (here is a link to their paper). If it is correct, then it is quite likely that their theory applies to Rainbow Boom/Rainbow Bust as well.
One last note: Akiyoshi Kitaoka has created many wonderful and novel variations on the Pinna and Brelstaff theme.