The angles of the incident and scattered x rays are constrained to be always equal. Dragging the mouse outside the crystal causes the angles to change. At about 37 degrees, the scattered waves are all in phase with each other. This angle satisfies Bragg's Law.
The incident x ray scatters from a single point in each unit cell. These points are all equivalent, and the scattering x rays will be indentical. Dragging the mouse around inside the crystal moves the points around inside the unit cells so that they are always equivalent to each other.
X rays are scattered by electrons, so the amplitude of the scattered wave depends on the electron density at the points. Electron density is largest near the nucleus of an atom. The atoms in the unit cells are at (0,0,0) (the corners) and at (1/2,1/2,1/2) (the centers), 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.