In X-ray diffraction the scattering factor f for an atom is roughly proportional to the number of electrons that it possesses.
However, for wavelengths that approximate those for which the atom strongly absorbs radiation the scattering factor undergoes a change due to anomalous dispersion.
The dispersion not only affects the magnitude of the factor but also imparts a phase shift in the elastic collision of the photon.
In contrast to desktop X-ray sources that work at a limited set of fixed wavelengths, synchrotron radiation is generated by accelerating electrons and using an undulator (device of periodic placed dipole magnets) to "wiggle" the electrons in their path, to generate the wanted wavelength of X-rays.
In addition to this effect the anomalous scatter is more sensitive to any deviation from sphericity of the electron cloud around the atom.