Please kindly explain, page-42

When the pixels move to their highest position they compress air in front of the pixel to form the first half of a sine wave, when they move to their lowest position the opposite occurs and they produce the second half of the sine wave. They're trying to make the distinction between traditional speakers that push air according to the input signal as opposed to a pixel whose sole purpose is to compress air in a single direction, then stop and wait for the algorithm to tell that pixel to move in the other direction.