Models of describing phasons include hydrodynamic theory (which describes phasons as a continuous pattern of motion), and 'phasonic flips', where atoms collectively 'jump' to new sites. Hydrodynamic analysis of quasicrystals predicts that, while the strain relaxation of phonons is relatively rapid, relaxation of phason strain is diffusive and is much slower. Therefore, metastable quasicrystals grown by rapid quenching from the melt exhibit built-in phason strain associated with shifts and anisotropic broadenings of X-ray and electron diffraction peaks.