Explanation of this example of pseudowaves

The drawing represents an array of 25 x 25 lamps, each of which goes on or off at a certain frequency. The left column has a frequency of 50, the second column's frequency is 48, the third's frequency is 46, etc. From top to row 12 there is an increase of phase, from 13 down a decrease. Click the start button and the lamps will independent of its neigbours start rhythmically going on and off. All illusion of moving waves is in the head of the beholder!