The angle of the incident and scattered x rays is fixed. Dragging the mouse across the figure moves the point around inside the unit cell. X rays are scattered by electrons, so the amplitude of the scattered wave depends on the electron density at that point. Electron density is largest near the nucleus of an atom. The atoms in this unit cell are at (0,0,0) (the corners) and at (1/2,1/2,1/2) (the center), so the amplitude of the scattered wave is largest at those locations. Note that the amplitude of the wave scattered from (0,0,0) is larger than the amplitude of the wave scattered from (1/2,1/2,1/2), indicating that those are two different kinds of atoms.