The thaumatrope is a toy that was introduced in 1825. There is a picture on each side of a disc. When the disk rotates quickly, which is generated by pulling twisted threads in the horizontal axis, both images are superimposed and appear as a single image. Probably the oldest example shows a bouquet of flowers on one side and a vase on the other; this is what my example is based on. When the disc is rotated quickly, the bouquet is in the vase.
What can you do? The thaumatrope is simulated in that the first image shrinks vertically and then the second image increases vertically. Then it is the other way round, the first image increases and the second image shrinks. This creates the impression of the disc rotating. You can increase the speed of rotation with the speed (flipping=1/speed) slider.
For comparison, you can also show the two images as a stop-motion animation (press the S key). in this case, the two images are simply shown alternately. You will probably then see the overlay effect even more clearly. You can switch back to the thaumatrope form of display by pressing the T key.