Layer 2 tells how to move pixels. Middle grey is no change, dark gray (down to black) is negative change, light grays (up to white) is positive change. As you may have seen there are two modes: Cartesian (there is a layer for X and one for Y) and polar (a layer for the radial move and one for a rotation).
In your case, the map is such that anything on the central rim is move dto the center, and anything between that rim and the middle grey area is moved proportionally towards the center to stretch the contents. If you add a layer of concentric circles to map the "whirl", pixels ill be dragged CW or CCW depending on their distance to the center and this creates undulations.
In your case, the map is such that anything on the central rim is move dto the center, and anything between that rim and the middle grey area is moved proportionally towards the center to stretch the contents. If you add a layer of concentric circles to map the "whirl", pixels ill be dragged CW or CCW depending on their distance to the center and this creates undulations.